OK liz. So, as per boontling, it would seem that it was used much more for covert conversation than for everyday speech, and I'm sure there might have been a word for “Ditto” or to agree, but it was probably so seldom used that it's hard to find a harper who's still alive who knows it. As close as I can come, from a very well versed codgy is the word “Yibe” which is a contraction of the phrase “You bet!”
So someone in agreement (ditto) would most likely say “Yibe to that!” which is definitely close enough to ditto for us boonters!
So I'm going to put together a small primmer on boont harpens that I will post in the off topic section. You have to read it, as that way you will hear the fantastic events I encountered to answer your question. m.