Jon,
There were 2 earlier drawings, both of which are said to be in the typical style of one open end veranda and one closed end. (ref Midland Wagons vol2 by Essery)
Drg 111 was dated 1875, and then Drg 302 in 1877.
I do not know if a copy of 111 exists (haven't checked the NRM list in detail). My list tells me that there is a copy of 302 in the Midland Railway Study Centre - if that is correct, then it could be a test piece for the new scanner which is being delivered on Friday!
I would then be able to identify the differences to Drg 753.
Andy