Block Casting Code (CTY)

Strange question.

We've got a SBC 400. SBC crank, SBC stock dished pistons, SBC rods, SBC heads (#461), SBC cam/lifters/pushrods, SBC intake, SBC headers, SBC fuel pump, SBC water pump, SBC timing chain & cove, SBC oil pan, etc. Fired it up last weekend. Sounds good. Pulls 16" vacuum. A timing problem (pointer doesn't match damper) but that's to be fixed by finding TDC and re-marking pointer. In short, it's a small-block 400 beyond any measurable shadow of a doubt.

Here's the mystery, at least to me.

On the block is a casting number, and at the end are the letters "CTY".

Everything I've looked up matches the letters "CTY" with a 396 big-block from around 1970.

????

Just curious to see what the block's original use was.

Thanks.