The kit sold for $800, which I think is the right price given the following pros and cons...
PROS:
Chrome in good condition.
Smooth bearing edges.
Probably the original finish, as the shells have an "M" stamped on the inside. Apparently Slingerland used to designate shells due for a wrap with "P" (for Pearl) and those due to be stained, laquered, or painted with "M" (possibly for Maple or Mahogany). The wrapped kits were worth a bit more originally. Occasionally both M and P will be stamped on the inside, which usually meant Slingerland intended the shell for an unwraped kit but then wrapped them even so. Anyway mine all just said M.
Probably the original wooden hoops- As I mentioned there are pictures of similar wood hooped Slingerlands.
Probably only one extra hole (still a big issue but slightly better than having 5 extra!) Basing this on pictures I found of similar kits having a four-bolt rail mount that corresponds with my mystery holes.
CONS:
The finish on the mounted tom has a reasonably deep gouge in it, which means refinishing absolutely perfectly would be impossible.
The extra hole and the 2-bolt replacement rail mount.
So that's it. 27 years under my care and now it's on to somebody else. I'm glad to see it lives on...
I extend many thanks to all those who responded- this site has been very helpful!