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Could be a ribeye cap, but it's hard to tell- this sort of nickname tends to be applied pretty inconsistently.
It's impossible to say for sure.
Standardized nomenclature for beef cuts was not agreed upon until 1992 between US and Canadian meat industry associations and there were regional differences in beef cut names as well as styles of butchering.
To this day, butchering in other countries often results in different cuts. French butchers process beef differently than Argentina, Germany, the UK, etc.
Today, there is the USDA's IMPS (Institutional Meat Purchase Specifications) guidelines which are voluntary and are intended to reduce confusion in big volume purchases (like government agencies, schools, hospitals, etc.) as well as cross-border trade so a porterhouse steak is the identical cut of beef from the same exact location and is recognizable as such.
Way back in the 1950s, US butchers did not cut their meat the same way and often there were various regional differences. A classic example is the tri-tip roast which was mostly confined to Central California in the 1950s but started getting more attention elsewhere in the USA since the turn of the millennium. Before, this cut was not marketed as a standalone roast, but further butchered for steaks, ground beef, etc. Thus, even today many people in middle America and the East Coast have less familiarity with tri-tip than those west of the Rockies.
So what was that cut your mom cooked? There's no way to know unless we could see a good photograph of the cut or read a detailed description of how the butcher actually cut it. There's a chance that there is no contemporary beef cut that is identical to that piece of meat your mother cooked 6+ decades ago.
In absence of a photograph or detailed description, my suggestion would be to go with one of the cuts from IMPS item number 114:
which would typically be marketed as "chuck roast" in a meat display case. Note that these various items are indeed the front end of the beef
and coming from the tough shoulder area require slow braising to tenderize the meat, the exact cooking method of pot roasts.
As pointed out earlier, the IMPS is a voluntary standard. There is not preventing an artisanal retail butcher from doing custom processing and marketing those custom cuts with fanciful, non-standard names. Of course, your mom's butcher could have been doing non-standard cuts or doing a standard cut but applying a vague name that was regionally used.
Even if you knew exactly what that Fifties era cut was, if it isn't currently marketed, you'd have to A.) find a custom butcher shop who would be willing to do the custom cut (undoubtedly expensive), or B.) adopt a similar common modern commercial cut (cheaper).
Anyhow, best of luck.
For reference, here is a list of alternate master names for common modern beef cuts:
Note that this list does not refer to archaic cuts or names and is likely incomplete.
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Boneless could have been tenderloin end. Grew up in the 50's and Mom made pot roast usually a big piece of chuck. Saturdays we had round steak. There was never a reference to End of Steak.
How did she make it? Spices, herbs or vegetables?