In the 50s and early 60s the issued revolver was Colt 4". In the mid-late 60s and into the 70s the issued revolver was a S&W Model 10 4" with pencil barrel. Issued holster was the Hank Sloan model by Bucheimer like the one pictured below with the Model 66. The Hank Sloan was pretty fast but didn't conceal very well. This one had the modification which included the cover over the hammer. Without the protective ear the hammers would eat the heck out of a jacket within a day or 2. Prior to the protective ear it was very common to put a pencil erasure on the hammer to protect you suit jacket.
The 2.5" Model 19, like the one pictured below, were probably the most common personal purchase firearm. The smiths at Quantico did a great job tuning this one.
Other common personal purchase were J frames (like this blued 49 below but the grips have since been replaced. The Quantico smiths put a great trigger on this one which breaks SA at 2 lbs and DA at 8 lbs), some Colt Dick Specials, and 4" Model 19s. A few had Colt Pythons but that was pretty unusual. I knew a couple who bought 3.5" Model 27s but it didn't take long until those agents bought something else. Just too much steel to carry all day on a dress belt. However, I knew 1 SA who carried his 3.5" Mod 27 almost his entire career.
Every field office had at least a couple of 5" Model 27s which could be issued out. Also there were a couple of 1911s in .38 Super which could be issued out for SAs working undercover. Hoover didn't approve autos for general carry as he viewed autos to be guns that criminals carried.
The 158 gr LSWCHP+P came on line about 1975 plus or minus a year. The manufacturer was W-W and for a couple of years Federal got the contract. Prior to the LSWCHP+P the issued round was the 158 gr LRN.
The Bureau also had .357 ammo for issue if an agent carried something in .357. However, even if carrying a .357 gun it had to be loaded with the .38 LSWCHP+P and the .357 ammo was to be carried for reload only. The .357 ammo in the 60s-70s was either Remington or W-W 158 SWC (non-HP). I seem to recall that Federal may have also gotten the contract one year but it was still SWC non-HP. I always liked the W-W .357 better as it had a sharper shoulder and larger meplat.
The picture shows the 3 issued loads. The one on the left is the Remington .357. The one in the center is the W-W .357. The one on the right is a W-W .38 LSWCHP+P. The Federal .38 had the same bullet shape as the W-W but the lead seemed to be a bit softer.