Thanks Bruce and Randy - I just found a very recent e-bay sold listing of a model 10 that was stated to be 1947, and it had the exact same cylinder configuration. Interesting, because it also had the later dark blue base with the lamp on the base, and a non-riveted boiler, but it also had the earlier Jensen decal. I believe that it was probably earlier than 1947, but it suggests to me that maybe they were using whatever parts were available in production, maybe using older parts if they ran out of the more recent ones? Regardless, all things considered I believe that Randy's estimate of late 1930s is probably correct (I see that you have a lot of Jensen engines!), and if you have a couple just like mine, then it is obviously a known quantity.
Any suggestions on the best way to clean up the stains on the blue paint?