Is it a Prentiss? Yes and no......but mostly yes.....except for the sort of no. That crazy bunch at Prentiss years ago did some funny marketing. They made the Prentiss line complete with markings and some extra bracing in their molds. This is the Prentiss line of Iron Workers and Bulldogs etc...that we mostly know. They charged $**.00 for these vises.
They also cast up an unmarked line of vises with not one maker mark on it...save the main nut (sometimes). If you look at these swivel model vises, they have the exact same "bench" numbers that a marked Prentiss has. These numbers were like serial numbers (sort of) and were stamped on the swivel jaw, and the main body. That way, once fitted, the jaw and body stayed together. If you look at these unmarked Prentiss vises, they will not usually have exactly the same casting webs near the slide support where it meets the main body.
Don't forget that Prentiss also made up Monarch brand vises and even cast in the PV Co into them. I have a catalog somewhere that has a display rack that Prentiss offered dealers and could be had with the Prentiss logo on it for the marked Prentiss brand and a "generic" vise rack for unmarked vises. The idea was that a store like Coast to Coast or Ace or whoever back then, could sell a cheaper vise for the cost conscious folks and it wouldn't have the Prentiss warranty, name, etc. You could place it next to a Prentiss rack if you wanted to show the stronger body and why you should pay more for a Prentiss.
Both are made by Prentiss, the parts are interchangeable, the designs are the same with the exception of some casting webbing. They were asssembled by the same folks using the same serial "parts mating" numbers. They were just made to be maybe 10% cheaper or so to still make money from customers who wanted a less expensive vise.
I will toss in some photos to show you what I mean.
Craig
This is a pair of numbers...the main body will also have a matching number. The number for this vise is "80". The slide, swivel jaw, and main body will all match from the factory on both marked and unmarked logo vises.
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These main nuts are interchangeable. One is from an unmarked Prentiss and one is from a marked logo Prentiss.
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Look at this unmarked Prentiss. Notice where the slide goes into the main body at the junction with the slide support. There is no "web" like a gusset there.
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Look at the same location on this marked logo Prentiss. See the "web" cast in? It is just in front of the "K" in York or the "2" in Model 22. It is a gusset to tie the slide support to the main body. All my logo Prentiss vises have that gusset. None of the unmarked ones that I have seen have that support cast in.
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All this is to say that Yes it is a Prentiss vise. It just wasn't a "logo" line Prentiss vise. It was an "in house" line of Prentiss vise. All the parts will interchange. Grats on a nice 5 inch Model 21 Prentiss vise!!
Craig