I believe that casting is used on all early big blocks. It is the same as on the 69 L72 427 I took apart today as shown here:
https://www.yenko.net/ubbthreads/ubbt...own#Post627507
The only way the poster is going to find the part number they want to see is on the GM box and it will have to be an early part as the part number was likely superseded many times.
Jason