To correct this I purchased Bronco's excellent Sd.Ah 52 flak ammo trailer. There has been a lot of discussion online about what flak trailer went with what flak wagon and there have been comments that the Sd.Ah 52 was only for the 3.7 cm Flak 18/36/ etc. auto cannon while the Flak 38 vehicles used the Sd.Ah 56 or later 57 trailer. While it is true that the Sd.Ah 52 trailer was the default trailer used on mounted 3.7 cm flak wagons, it was also used on the Flak 38 vehicles due its large size and need for lots of ammo for its quad auto canon.
There are a few distinguishing features between the -52 and -56 trailers but the most easily recognizable difference were the wheel rims.
Here's a picture of a mid or late Sd.Kfz 7/1 Flakvierling 38 towing a Sd.Ah 56 flak trailer. Note the convex shape of the wheel rim and bolt sequence on the hub. Also a picture of just the trailer itself.
Now, here'a picture of the Sd.Ah 52 trailer.Note the flat face of the rim.
And here are pictures of some early flak 38 vehicles towing the Sd.Ah 52 trailer.