Yes the fret board is very flat and to look at you would definitely assume its a nylon string classical guitar. Actually in my research I've found a couple of different forum discussions regarding this. As Simon mentioned above they only started making steel string instruments the year before this model came out, so perhaps they were still finding their way.This model was also very short lived and obviously the FG's are much more in line with what you would expect from a steel string acoustic.

I think they looked at their classical acoustic and said, "wonder if that would work as a steel string with some minor mods?" The G-50A is the nylon equivalent.

It is a difficult guitar to properly research as there isn't a lot of definitive information, so a lot of the info is second hand.

This guy seemed to know his Yamahas on one of the forums I read through:

"The Yamaha S50A is a Steel string folk guitar which is often mistaken for a classical guitar, putting nylon strings on it is a big mistake, it was not made in 1961 as someone on here claimed by it's serial number, it was infact made between 1969 and 1972 as Yamaha didn't start making guitars with the tuning fork logo on the head stock until mid '69. This model is quite rare, I have one here and the sound.."magic" For a mint condition one you shouldn't accept less than $600 US, for a rough one you should still be asking atleast 250 bucks as age only makes these guitars sound better no matter how rough they might look. I also agree with someone else's comment on here that Yamaha do need to get off thier asses and make us a list as everything currently available is useless. Hope this was of some help, thanks- Wade"

I've given this one a light clean and put a low 'e' string on it. The action is pretty high for my tastes, but it does play clearly and ring out nicely. I'll get a full set of nylons and see how that goes, I'm wary of the amount strain steels would put on it. Shame there isn't an easy way to adjust the string height- I don't really want to pull it apart too much. I'll post some more pics later on of where its at.