Ah. Another interesting theory... That could make sense. Maybe they were going to go that direction early on and then had to change the design. No reason to spend time and money changing the side castings again if you could make them work. The more and more I think about it, the more I think you are correct. It sure looks like it is set up almost exactly for that.
I did try looking up Aspens on websites from other countries. Most of what I found looked to be an earlier version of the Aspen with an ash pan, like the 1920. No parts diagrams, and maybe the pictures are just 10 years old on those sites, too, so we may not know for sure unless someone from outside the US/Canada has a recent one and can chime in.
I think in the spirit of maintaining the gasket in the configuration that the factory seems to have had it on at least 3 forum members' Aspen C3s, I will leave a gap on the top of both sides as I put it together. It really doesn't look like it burned out of those spots. It seems like it would be simpler to tell your factory floor assemblers to just put gasket all around like they did on every other part, so it added some complexity to the assembly process. VC (or whoever owned them at that point) likely had some reason.
