440 HP block stamping ?

[73-charger-383]

Scored a free 66 (10/25/66) 440.  It's a HP block and has C440 HP  and then 12 21 underneath that stamped on teh pad.  Any idea what the 12 & 21 stand for?

[73-charger-383]

Casting Number is 25364307

[John_Kunkel]

12 21 is the build date December 21, 1966. It's a '67 model year block.

2536430 is the basic casting number for a '66-'72 440, the -7 is a foundry casting core number.

[73-charger-383]

Thank you!!