"They also have a sister museum in Speyer, not far away."
Yes and I went to Speyer on the same trip as when I visited Sinsheim. Another excellent museum and here's a link to a photo I took there of an Antonov 2.
JetPhotos.Net Photo » RA41343 (CN: 1G 65-18) Aeroflot PZL-Mielec An-2 by Michael Blank
And "My theory at the time was that the Russians only had aerial photo's to work from."
That sounds entirely feasible to me.