Can anyone tell me which model of Singer this is? Based on serial number it shows to be a Series 31 but I've bought numerous bobbin hooks for a 31-15 and they are slightly too large and are binding when turning the hand wheel. Also recently bought a Series 15 and it fit too large as well....almost EXACTLY like the "31-15"'s I orderd. Maybe I was sent the wrong parts originally...not sure...... Any help would be appreciated. TIA