Had to replace one of those not to long ago, as one of the breakers would no longer re-set.
It was mounted in a residential kitchen, recessed into a brick wall, and fed through pipe. As you may have guessed, replacement breakers are as rare as hens' teeth.
With this type of enclosure there are no buss bars; the breakers are wired direct, and mounted to the faceplate. Square D does, however, make a "feed-thru" type breaker, with clips that allowed me to mount it to a new fsceplate that I made.
Took a lot of time, and cost the customer (maybe) $150 for replacing a breaker....but there really wasn't much of an alternative.