I stand corrected the number as i can make it out is 18RC085 it is hard to tell as its been powder coated. This is the problem with it sitting in the garage for years even i have forgotten what size it is. so definitely an 18" then?
I could be interested in this - I live locally to you but not home for a couple of weeks. Can you measure the distance from top of saddle to top of pedal with saddle where it is in the photos and pedal at the bottom of the stroke?