What does it mean?

Mopar "B" engines are all "low-deck" being the 350, 361, 383 and 400. The "RB" engines were the 413, 426, and 440 meaning high deck, ALL are "Big Blocks" just the intake height is different requiring specific intakes for the 2 types. In 1959, Mopar (Chrysler) had to figure out how to change between engines types in a timely manner and had for that year BOTH a 383 "B" block as well as a 383 "RB" engine, also said above.

"B" blocks are Big Blocks, and "RB" "R"aised "B"lock engines. As mentioned the RB's had a longer stroke, where-as the B engines had a shorter stroke.

Then, you had the "A" engines, Mostly Poly-Head engines (special combustion Chamber, Semi-Hemi), heavy cast blocks. BUT, then you had the "LA" meaning "L"ight "A" blocks that were the 273, 318, 340, and 360.

The Slant 6's somewhat followed as the "LG" 170 "L"ow "G" block. The "RG" 198 AND "RG" 225 were "R"aised "G" blocks.

The early true Hemi's were A blocks (I THINK) I haven't seen any "letter" destination on them, BUT that changed when the 426 Hemi came along as it was an "RB" Hemi as the second generation Hemi engines unfolded in '64. The 426 Hemi was built off the existing RB 426 Wedge-Head engine.

The Early A blocks of the Poly-Head design, also could have had special ordered Hemi Heads bolted onto them. Replacing the Polyspherical heads, to make it a true Hemispherical head.