It is an early 500-series and most likely a 510, 511, 512 going by the gray trim on the rug nozzle, the gray plastic handle grip, hollow handle fork, and the old-style toggle power switch.
Earlier models had gray ribbed-rubber handle grips and gray trim; the 505 and earlier had black trim.
The 513, 514 and 515 had red plastic handle grips with flip-down cord releases, solid handle fork, and the "push-push" power switches.
So, that would narrow it down, most likely, to one of three models.
The front hinging part of the light housing has been broken off somehow, and the electrical cord to the light socket is gone. I've NEVER seen this in almost 40 years of collecting!
How it got broken is a real Catholic School Mystery -- it would take incredible force (or sheer will) to remove that hinging part since it's held in place by two very stout rivets. It would have been far easier to have removed the entire light housing assembly since the rear part of it is just secured to the motor housing with two screws.
That is a replacement bag. That type of gray specked bag was available as late as the mid 1970s, although by the time the Sanitronic came out, factory-rebuilt machines were done in tan and then avocado. Later on, they were done in Classic III red, then, I believe after that it was back to tan. I've also seen a couple of factory-rebuilds in dark brown and dark blue but not very many.
