I know that in teh 20's they were starting to drop the gravity of beers, and introduce the use of cheap refined sugar as an adjunct. . . . it's just that 17% is high by any standard (Possibly not Belgian triples though) and only 5% crystal . . . colour looks about right but that is what the black malt is contributing.
I do suspect that it will be a bit on the thin side for a mild of that era though.