I don't know about the correlation between frame numbers (if there are any!) and years of manufacture - you will have to google this. The best way to tell the year would have been the engine as they used several (mostly small industrial units) over the almost three decades this thing was built but the engine is obviousely gone.
What's it worth? No idea, but I wouldn't expect too much from it! Best way to find out is to put it on the market somewhow (ebay?). This way you don't have to find out more about it as the potential buyers/bidders will know more anyway.
But maybe it carries an emotional value for you being your late grandfathers ex-mount? That's to decide for you alone! In this case keep it, get information, find an engine, put it all back together (it seems quite complete apart from the missing engine) and have (if not too much, I'm afraid....) fun with it!
Cheers
Andy
