Mike,
How long is the bar on your DB? I believe mine is an original also. Does yours run? Mine runs but I really haven't used it much, it's REAL loud and HEAVY! I haven't heard of any other units other than the two that I have, I really don't know much about them, if you know any history or anything else let me know.
As far as the cap, my saws are at my dads house and he lives quite a ways away, as soon as I'll get over there I'll snag it for you. I'll get your address then, you can have it, I don't believe I'll ever need it.