butlersrangers, thanks for your reply (and the welcome note).

It is definitely a flat side plate, without the door -- the stock Dad carved from a chunk of Maple covers it.

I'm pretty sure there's no issue with a magazine cut-off switch, but I'll double check.

Wow, do I feel dumb now! I think "sliding in from the front" rather than "pushing down from the top" is the key. The tips of my "too long" cartridges were catching at the front of the receiver, after I had partially inserted the rim as far back as it would go. Putting one nose-first partially into the chamber and backing it into the magazine is the obvious (now that you point it out) solution. After all, they fed from the magazine into the chamber smooth as butter once I got them in there and cycled the bolt.

Thanks, man!