1885 is what my book says. 38-40 is basically 44-40 necked down. Takes
a .400-.401 bullet.
I've got a later (1914) '73 in 38-40 that shoots surprisingly well with
almost no rifling visible.
30" barrel? If that is indeed the case, and it is octagonal---you
appear to have a special order gun. Don't do ANYTHING to it.
A little wipe down with a slightly oily rag is OK---but don't
"clean it up". Get a "factory letter" on it--that long barrel could be worth
some serious bucks.