"T" or "P" by engine block casting number?

[maroman]

The red SS '67 Camaro that Hot Rod tested for about 6 months was another. Hot Rod admitted at their 75th ann. That the car actually had a built 427 and all the time slips were trimmed.

[JKZ27]

I know this discussion is about the T and P but may I ask if anyone knows what the M in a square might be? I have a Flint 327 block and this water pump showing the M near the casting number.

[ZLP955]

May be similar to the M blocks and heads:
http://www.camaros.org/drivetrain.shtml#BlockCast and scroll down to "M"-Coded Blocks header.
Seems those were evaluation castings rather than date codes, with some being released into production. I have a pair of M-codes 186 heads on my 302.