Taffer, it seems you're right!
I sat down a minute to do some relaxed filter screwing, and Yea! the yellow filter fits. So, I took off the collapsible hood of the M-Rokkor, and that one fits too.
Dang, it seemed I had a reason to not make haste with finding a hood, but 40.5mm hoods are easy! Will this lens come to Mongolia after all...?