Evening all...
This website,
www.planespotters.net/Production_List/
has a list of all the a/c produced, it gives N757A as the first airframe still with Boeing but the next three N501EA / N502EA / N503EA went to Eastern. However going by the first flight dates there was an 8 month gap between the 4th a/c (503EA) and the 5th (504EA) making it's flight which I reckon is your certification process. You couldn't certify the aircraft in 8 months with just the one jet.
The same happened I remember more recently with United and the 777. Only the first one was Boeing's to keep the others all got painted up into United's outfit although still flying for Boeing and used for the certification then handed over cheap to UAL.
Regards technical info the best I can do is the technical manual they give us lot when converting onto the 757 which I suspect may not be technical enough, see what you think if you don't have it already. There's all sorts of stuff here but it's a bit like charity shop not guaranteed to be the latest issue.
http://www.smartcockpit.com/plane/boeing/B757/
Hope this helps...
