Frank50's observations and questions about the stamped serial number are good. As I mentioned a few months ago, I own or have owned five 261 engines and only one of them has (or had) a stamped serial number that was correct for the block. The other four turned out to be replacement blocks, and the owner either left the pad blank or stamped in the number from the original engine (not a 261) that was in the car receiving the 261.

The engine in the 1949 Chevy in question seems to be a 50-52 PG engine, but there are other possibilities. the only way to know for sure is the take the engine apart and see what is inside. There is the possibility (close to zero) that a low pressure engine with babbitt bearings had been converted to full pressure with insert bearings. The block and head casting codes and date codes are needed, as has been stated above. Even these can be confusing, as the date casting code for a replacement part might not be consistent with the year for the original.

Again, thanks to truckernix for the link to the engine history summary.

Hoyt