This is the technical term. A strap is a fastening (with a buckle), but this is a decorative strip across the vamp (the instep). As Robin rightly says, the answers to the previous question (vamp or upper) are wrong, though somebody does mention the term saddle there in the discussion

This feature is characteristic of loafers, especially the classic penny loafer, rather than moccasins. In English moccasins and loafers are distinguished but in Spanish mocasín tends to be used for both. These are probably loafers.

A Google image search for "mocasín" + "antifaz" gives you lots of pictures of shoes with this strip, which make it pretty clear that's what antifaz means. The following comes close to an explicit definition:

"Quizá el nombre de penny loafer te parezca sumamente raro, y aunque éste sea tomado como un mocasín más, su característico detalle de tira de piel en el empeine le merce también el nombre de mocasín de antifaz."
https://www.trendenciashombre.com.mx/accesorios/elegantes-y-...

I think the term (lit. mask) probably refers to the typical diamond-shaped cut-out in this strip, which gave penny loafers their name (supposedly people kept a couple of "pennies" in the hole), comparing it to the eyehole in a mask.

Anyway, saddle is what this strip is called. It's a bit confusing, because saddle can also be a sewn part of the upper, as in two-tone saddle shoes or saddle Oxfords:
https://en.wikipedia.org/wiki/Saddle_shoe

But in loafers it's the strip, the "antifaz":

"Sometimes loafers feature a piece of leather across the vamp, which is known as a saddle [...]
Wildsmith loafer in brown with Moccasin construction and typical saddle [...]
The Penny Loafer
In 1936 (some sources put the date as 1934), the G.H.Bass shoe company introduced its version of the loafer, and the company is known for it to this day. Their design included a distinctive strip of leather (the saddle) of the shoe with a diamond-shaped cut-out."
https://www.gentlemansgazette.com/loafer-guide-penny-gucci-t...

"Add to your work or formal attire with this sleek penny loafer shoe. It comes with a traditional penny saddle, apron stitching and round toe. The flexible sole completes this smart style."
http://www.dunelondon.com/riggs-penny-saddle-loafer-shoe-038...


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Note added at 2 hrs (2017-01-04 01:21:45 GMT)
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Do you mean they've told you to translate "antifaz" as "loafer", as in "It may have a loafer"? That's just plain wrong; in fact it's nonsense. A loafer is a type of shoe and an antifaz is a part of a shoe. Most loafers, though not all, have an antifaz, a saddle. I suppose they might mean that you should translate "Puede tener antifaz" as "It may be a loafer" (better would be "a penny loafer"), though there seems no good reason to do it that way. Are you sure they don't mean that loafer is the word for mocasín here? That is true.
In any case, please don't put "loafer" in the glossary as the translation of "antifaz", or it will mislead everyone in future!

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Note added at 2 hrs (2017-01-04 01:32:17 GMT)
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I'm not saying you shouldn't do as you're told, Eileen. If they're paying, they're entitled to demand that that translation should be nonsense if they want. But let's be clear: translating antifaz as loafer is not just wrong, but complete nonsense.

I leave it to you to decide what to do with this question. If you think your client is right, you'd better close it without grading, I suppose. Alternatively, if you think either Robin's answer or mine is right, you could take the opportunity to get a correct answer into the glossary, since we already have a wrong one.

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Note added at 16 hrs (2017-01-04 15:23:55 GMT)
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Re. Helena's comment. First point: I carefully explained that the saddle on loafers is a different use of the word from the one in saddle shoes, so that point is irrelevant. Second, other words are sometimes used, yes. Some manufacturers do refer to "straps" on loafers, but they're quite often referring to a fastening strap, or at least to a decorative strip with a buckle that looks like one, and which is not an antifaz. "Strip" is better, but it's not specific enough; it could mean various things. "Mask" is calqued from Spanish. "Penny holder" is occasionally found but only for the exact original penny loafer type of saddle.

I looked at and weighed up all these options before answering, looking at frequency and exact meaning.

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Note added at 16 hrs (2017-01-04 15:29:33 GMT)
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Look at images for "saddle loafers". You won't see any golf shoes.

Next should be pretty reliable. See here saddle loafers, snaffle loafers and tassel loafers. The first are mocasines con antifaz; the pictures are exactly the same.
http://www.next.co.uk/shop/gender-men-productaffiliation-foo...

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Note added at 16 hrs (2017-01-04 15:36:28 GMT)
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So, as I say:
Strap usually implies with buckle
Strip is descriptively accurate but too vague and not used by manufacturers (loafers with strips?)
Penny holder is pretty rare
Mask is used only by Spanish companies, as far as I can tell, for obvious reasons.

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Note added at 16 hrs (2017-01-04 16:09:37 GMT)
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Or try Wikipedia on types of loafers.
The Aurland (1930): "Raised seam on upper, similar to moccasin. Narrow cut out on saddle."
The Penny (1936): "G.H. Bass of Wilton, Maine launched a loafer called the ‘Weejun’. It became very popular in the U.S, especially among prep school students, who kept pennies in the saddle slot for pay phone calls. Hence the name ‘penny loafers’."
https://en.wikipedia.org/wiki/Slip-on_shoe#Types_of_loafer

Aurland loafers:
https://www.visitflam.com/en/aurlandskoen/

Penny loafers everywhere.