Jim, the characters stamped into the block are the best way to identify these engines. See attached page from the '74 Chilton repair manual Jeep chapter. This shows the 1967-71 Buick/Jeep V6 codes. Another site, www.earlyCJ5.com indicates there is a third letter in front indicating Domestic/Military/Export. Then there will be 3 numerals at the end for date of production. A Buick V6 of the '60s built for car usage will have only 2 letters then 3 digits. Those engines can be decoded from the Reference section on this site.
The assumption so far is that the engine dates from this period. The manifold casting number looks close to other Buick casting numbers of this time. These early V6s use a distributor with breaker points. The cap will be the odd-fire design that looks like an 8-cylinder cap with 2 of the posts blanked off. Engine will not have much pollution equipment on it either. Does this sound like yours?
It could also be some later Buick V6 from a car. Features would typically be HEI pointless electronic distributor, regular even-fire 6 post cap, more pollution equipment. Although it might not be original to the engine, we can examine the distributor part number.