The paper label in your Washburn was used from 1906/7 - 1915. The serial number A3991 would date your guitar to 1907. It's hard to quit make out but it appears to be a style 115 (see bottom of the paper label). The bridges would typically be Ebony and normally have flattened pyramid wings, this one looks more like one you would see on a Regal made guitar. They also owned Regal Instrument Co. and there was a lot of parts swapping done at that time.
Here is a picture I had handy in my computer from a vintage instrument exhibit we hosted in 07 of my Style 115 Washburn. It has the Ebony, flattened pyramid bridge I mentioned.
This instrument would have been built for gut strings originally. It will be ladder braced and have a bridge plate that is in all likely hood be made from Spruce. The straight across, non-compensated saddle will not play in tune with steel or silk and steel strings. I use Nylgut 'Alabastro' strings on mine and they sound and play great, Elderly Instruments has them. Any plastic strings for classical guitar would work though.
Washburn serial number search up The Ukulele Hall of Fame Museum - 2008 The Ukulele Hall of Fame Museum is a non-profit organization dedicated to the preservation of ukulele history. Ricoh aficio mp c3300.