Pad looks good, btw.
1) The 'stamping' itself 'looks' good, but I understand there are 'restampers' who can duplicate original stamps/holders?
2) The original broach marks are not visible in the supplied photo, but all kinds of horizontal and diagonal lines are very visible, which is suspicious.
3) The engine assembly date of 0724 is more than 2 months after the 0522 cast date, which is very unusual, and also suspicious, in my experience.
4) The VIN provided of 687352 could not be confirmed from the pix (by me), but that VIN corresponds to 18-19 September 1969, which is a day or two after my Z28 was built. This also raises suspicions as my engine was cast on 0826 and assembled on 0827.
I'd have to see a LOT more information and examine the engine before I'd feel good about this being an original DZ block...