Somewhere between serial numbers 1120, Mason and Hamlin abandoned the screwstringer design and went to more traditional tuning pins and pinblock. (I'm so sure of this, that I'll bet good money that it is!). Your instrument is almost certain to be a 'screwstringer' design. However, I'm going to disagree with the claim that old Masons have more value than standard pianos. I'm going to also agree that this is an old M&H - based on the serial number you indicated, it's from one of the first years they manufactured pianos. Anthony's answer that you should have a technician inspect the piano.
