Youre dead right, Murray, that sevenths on the tonic came in quite early in the history of harmony....but that just reinforces my point doesnt it? Because they had a flat seventh as well: so all the sevenths in use had flat sevenths, so they called them all sevenths.
I've no idea when terms like C7 came in. Late 19th century, at a guess? I've got a lot of music books, but I dont remember seeing that notation in anything pre-19000. Before that harmonies were related to the key you were in :II meaning triad on the second of the scale, and so on. Dominant sevenths were written down as V7. (I'm talking of the English/European stuff I'm familiar with, I've no doubt there are regioanal variations.I've been playing with Arabs, French, Kurds and Armenians recently, andnone of them use C7! They all write do7.