What year were Marlin 336-A rifles first
drilled and tapped on top?

1956. Before that they had a "waffle top" anti-glare pattern on the top of the receiver.

Thanks,
This one has an "H" prefix which predates that. Looks so professionally done that I thought it was factory. In my younger days I would have returned it but what the heck, with just a few left what does it matter? I still have a nice .32 Secial that I always wanted. Thank you so much for your help!