Think you are right, oblong block with a hole for valve to go (possibly rubber bung in block hole), the plate is what the block 'slides' against and...
I like the design of pumpers but I imagine most people don't find them that practical in comparison with modern stuff. I have 3 older pump air rifles...
Download 'From Ingot to Target'. Bullet casting lore going back to before time began. I've been casting since the seventies and still learnt lots of...