Mesa doesn't operate the 170 for UAL. However, I looked in apollo, and UA
flight 7192 on those dates is indeed listed as a 170, and 7192 would be a mesa flight number. This is probably an oversight on someones part.
The only carrier that operates the 170 for UA
is Chautauqua (or Republic, which is the parent company). To my knowledge, no one else is or plans to. Shuttle America only operates the Saab 340s out of IAD
This is a very good find though and I think very intersting to say the least.