I was at the junkyard the other day and saw a 360v8 with a mirror image "150" on it where you would normally see "352". its cast into the block front and back just where i expected to see 352.
The reverse "105" blocks were cast at MCC (Michigan Casting Center). So that is simply all it means. The "352" blocks were cast at DIF (Dearborn Iron Foundry) If you look closely at each block you'll see the foundry logo's as well.
MCC came online I beleive late 71 to early 72 and took over production on most FE and FT blocks. However the 427 and 428 remained cast at DIF till 73 and 74. After that MCC took over all block casting (except the 427 which was out of production) till the end of the FE's production. For a short time in 68-69 the Cleveland Foundry CF cast FE blocks. Many FE service heads were cast at CF after DIF production runs were thru. Most of the hipo heads were still pretty popular and cast well into 75 there. Sorry for the long winded answer.....