John,
Yes, number on the left when astride the bike. An example form one of my frames:
You can use this to check the fonts against what is stamped on your frame.
Douglas did use "EXP xxxx" as a part numbering system for experimental and pre-production components as evidenced by references on some of the factory drawings to other parts or the use of stampings. For example 'Use stamping EXP xxxx until suplies run out'. I cannot think of having seen an example on an actual part. Indeed, I have only seen a part number on a casting when it was worked into the pattern, if even then. I've not seen a part number stamped. Douglas used a chronological numbering assignment for part numbers. So numbers assigned to the Blue Chief are seen from mid-16k to 17k range. Much of the Aero part numbers would overlap as they are likely the same components as the Blue Chief (nuts, bolts, knee grip plates, etc), excluding the parts they actually did change. So they run from the upper 16k range to about 17k as well, and the 1937 models run into the 18k range. I am not sure exactly how the experimental numbering worked. During the DT era the experimental numbers seem to have lagged behind the production numbering. Not sure if they have their own numbering pool, or (most likely) if it was just simply because the experimental number had been assigned months or even half a year prior.
But the number you found on the bottom of the frame is most likely a component number. It is about in the right numeric range and too many digits for a frame serial number. Nor have I heard of them using EXP as a prefix for competition machines. They just used the same or the next assignment of their standard frame numbering format (including an "F" for frame.) True one-off specials are more likely than not to have no number.
If another number does not turn up, then my guess would be it is a Blue Chief frame that the factory built up or modified as a pre-production Aero frame. I don't know which side of the frame the Blue Chief was stamped on. If most were stamped on the left, then that would be another anomaly. On the F28, 350EW, and A31 they are stamped on the left, so I assumed that was the preferred side.
-Doug