The 20" has a 1977 engine, compared with 1979 for the 17", which means it is probably two years older. The reel and bedknife are in better condition on the 20" and look as if it may never have been resharpened. However the primary chain is shamefully loose, as well as dirty, rusty and unlubricated, and some of the sprockets are quite worn.
I am no expert, but that looks like an eminently restorable machine. I hope bonnar_bloke has time to take a look and put it into the context of his wide knowledge and particular enthusiasm for that model.
