The engine pad stamp appears original to me, font and characters look good. The "M" was probably added by an owner during a trip to the machine shop, I always stamped my initials in an inconspicuous part of the casting in case it got "destroyed" or "lost" during the machining process (I wouldn't have done this obvious a stamp location !) Maybe that sounds paranoid, and I guess it was, as I never had one turn up missing (but I've heard horror stories over the years, too). If the VIN stamp above the filter is a 6 rather than an 8 as it seems from a casual glance, that could be pretty close to the range. My Z11's 3/17 built block carries a 620 VIN -
Regards,
Steve