Both the Snorkel and PFM Demonstrators have Medium nibs. The BCHR and Jade Green Cutaway Demos are nibless while several years back I had Sherrell Tyree restore the Black cutaway and install the proper nib in it so it is completely usable.
I've got more cutaways but can't post them as my digital camera gave up the ghost last week and I am currently saving for a DSLR.
I have these two plus a cutaway pencil from about 1919. The Snorkel demo is perfect in every way except it has been inked. It has a factory stub nib. The Balance demo is from about 1936 or 1937. It didn't come with a nib but I put the appropriate one in it. They're both restored and working. I recently saw the same Balance model sold on eBay with a black cap. The seller claimed it was correct (to be fair I have seen the same set up in book). I have a hard time believing Sheaffer would sell it both with and without a clear cap.
I too suspect that the Balance Demo without the transparent cap was a "frankenpen". Given the tendency of the early balance demos to badly craze, I'd be willing to bet that is why the eBay pen had a black cap.
There's one currently on eBay that I'm watching just like mine except witha drilled out cap like your older ones. Interesting. I can't help but think its a shop made demo, the top at least.