Nice .38 Hand Ejector. Identical to my .32 "I Frame" (not an official designation) except for scale.
If I could afford it I'd like to have one Hand Ejector of each frame size, with an unaltered .455 British contract version being on the wish list.
The side sprung hammer block can be disabled by hardened fouling and grime, or rust. It can also lose spring tension, or very rarely be found broken.
Warping of the side plate from ham handed removal can also result in binding of the hammer block.
First thing I did with my Hand Ejector was to detail strip and clean every internal part, and carefully check the function of the hammer block.
"Pre-64 Model 70" is a commonly used term among collectors, so pre Model 10 M&P is fine with me, though generic and not very descriptive and it includes a number of variants.