That could theoretically be any model from 505 to 515, depending on whether it's been serviced or rebuilt over the years. The only way to know for sure sure is to ask the owner to look at the serial number tag on the bottom.
My guess is that it's a 510, 511 or 512. The bag appears to be in nice condition, just a little faded. It would be worth to get the machine just for the bag!