Hamfish,
I am familiar with these 'pillbox' conversions. Although they look menacing, the vast majority of those I have encountered are simply friction fit. In this type depressing the shitodome-ana 'plugs' and pulling on the gilt 'collar' will release the lining of the kashira. Putting your finger against the lining of the fuchi and pulling up will release the lining. Note the "plugged' nakago-ana of the tenjo-gane is usually soft-soldered in place. If this is the case repair is relatively inexpensive. NOTE, rarely the lining of the other parts are also soldered, this makes revision much more costly. If this was a GREAT piece in otherwise perfect condition, I have, and would undertake this project.
Cheers,
-S-
p.s.- these are collectable 'as is', as Stephen notes they are nice to use for their intended repurposing.