Dan San,
I don't think that the progress of changing crosses is the only way to determine the date of the mentioned photo. As far as I know all Fokker Triplanes were delivered with the out of date national insignia. Even new Triplanes that were delivered in April were so marked and were changed by the Jastas in the field.
Wenzl was there and he told his story. The book is very accurate concerning dates, losses, personnel and so on. Compared with other books written by german aces it has some historical source-value.
At the time he wrote the book many other Staffel-comrades were still alive. I never heard of any complaints from his former JG 1 mates that his story was nonsense...
Wenzl was later in close contact with Bodenschatz. Bodenschatz used Wenzls photo when he wrote "Jagd in Flanderns Himmel", the war diary of JG 1. Bodenschatz repeated Wenzl's story about the joke with Moritz and Bodenschatz too was a witness of the morning hours of 21 Apr 1918 at Cappy airfield. I don't think he would had written this, if it simply wasn't true.
Once more: the line with the wooden block which was tied to Moritz' tail is on the photo!
Immo