Reinforcement plate - not sure if that is stock or not - but if you are familiar with these spurs, it is quite obvious why they are there, as these (little round barrel type) spurs are notorious for stressing the shell. It is even more so important to have reinforcement when you have thin shells - like you indeed have there (hence the reinforcement rings).
BTW - alot of MIJ shells have the type of construction that benefits from reinforcing the spurs (of this type) and I usually try to install some type of reinforcement on mine - usually in the form of a backing plate.
Strange how....I suppose it must have been some kind of trend to paint the interiors of BDs black. I suppose this was in the days when it was more common to run without a front head, and perhaps the black paint job was simply for aesthetics.
If I were you, I would not bother to do anything about this black paint - just leave it as is....and run a front reso head - and it will not been seen really. I see you are already up on good heads (Super Kick) and the fact that you have wood hoops is also very good for achieving good sound out of your kick. 20s used to suffer from a reputation for lacking lows - but with our improved head technology / general drum knowledge, they can produce better than some 22s/24s, etc.
In terms of what to do with sealing the shells and stuff like that. I have several of these MIJ drums and have not sealed any of them. I do not feel they need help with 'projection' and their condition seem to be ok these 40 years later. I make a point to keep the humidity between 40 and 60 percent and they keep just fine.
Good advice above in terms of tuning approach and so on. Egdes, heads, tuning. My specific advice is to tune to the shell and tune both heads the same - reaching maximum resonance, then muffle from there. I use EQ rings.
Enjoy....as it looks like you have one of the better shell type and good looking MIJs out there!
Bop iT / Til U Drop iT.