Regular / F20 block dating

[Homitchell]

Have 2 blocks that I would like to know the approximate year of casting. Neither has a serial number stamped in the usual location. I am concluding they are replacement blocks.
USRC 423 DD 5-8-Q
423 DC K24F

Thank you for your input.

 

[Oldiron29]

Ho, I don"t have a listing for Q. But F is 1936
oldiron29

 

[robgIN]

I have a replacement block for an F-30 that is a V, which works out to be 1950, so Q should be about 45.