Nothing particularly special. It was introduced in 1946 and used in CJ's,d M38's and as replacement motors for WWII Jeeps. CJ blocks were stamped 2J (CJ2A's), 3J (CJ3A's). M38 blocks were stamped MC or RMC. Industrial blocks, replacement blocks and utility blocks had different prefixes. I've never seen "MMS" before, but I have seen California assigned VIN's that end with "CAL"
1944 MB 349858, 20620361
1942 MBT 10968 viewtopic.php?f=18&t=149552
Collecting Willys Engine Info. Please send me Engine #, Casting Date & Machining Date if known