Your question is valid. Your sources are the problem. If you look closely at the photo you attached, you will find a number of things that are wrong: The lettering is repainted, and in the wrong place, it should be lower on the projectile. Proper color codes for HEP-T projectiles are all black body (anti-armor) with white lettering for inert, all black body with yellow lettering for HE, OD green body with blue band for practice, or OD green with yellow lettering and black band for HE (anti-armor). The explosive fill on HEP-T projectiles is never Comp-B. HEP-T projos use a plastic explosive Comp A or Comp A3. When people repaint things, they sometimes fabricate their own model (M) numbers and lot numbers.
The next thing to consider, is that in some instances, a projectile body will be stamped in the body with one number, which is the part number for the body, and it will be assigned a completely different Assembled Cartridge number, which is the inked on lettering on the loaded shell. The lot number of the loaded assembly will be a different number than the lot number of the metal body and the lot number of the fuze or the lot number of the cartridge case.
I have examples of 75G, 76G, 90G, 105H, 105G, 105R and 106R (same projectile), and 165G projectile bodies. In addition the 105G WP-T projectile body is the same as the HEP-T but is stamped with a different number. The 105 G and 165G projectiles are more recently produced and were produced by Norris Industries (NOR). The 105R/106R body was produced in 1955 by Chamberlain (CC or YCC). It is stamped with a T number but assembly lettered with M346.
All of the rest of the projectile bodies were manufactured in 1952 or 1953 by Chamberlain. It appears that HEP-T projectiles were tested in almost every Gun or Rifle available at the time. Below is a list of Metal body part numbers stamped into the metal and the years they were manufactured. You will notice that the 90G body has numbers similar to what you have been told of the one you are thinking of buying which is very overpriced at $100.00 for a rustball;
75G T165E11 1952 (For Sherman)
75R T151E22 1953 (Recoilless)
76G T170E3 1953 (Walker Bulldog)
90G T142E3 1952 (Guns)
105H T81E28 1952 (Howitzer)
105R T139E45 106R T139E47 1955 (Recoilless rifles 105 and 106mm) Both numbers stamped on the same body.
Assuming you end up with a projectile, you should look for a case around 1952/1953 timeframe with the same date on the primer.
By the way, your title (M691 HESH) is a European Mecar manufactured round for a low-recoil gun. Not even related to U.S. Ordnance.