This is a 1940 Kay model S-51, originally a 5 string bass it has been converted to a 4 string, beautiful ambered-out blonde color, top model with one piece carved scroll, inlayed perfling etc., it has an incredible thick black ebony fingerboard, great setup, and sounds great.
"S" model stands for "Swingmaster" which is their top of the line model. If it was still strung as a 5-string it would be worth about $5000. The convertion was only by trimming the extra wide fingerboard to a four string width. A wide fingerboard can be replaced and it's back to five-string (same neck was used for 4 and 5 string models, just different fingerboard width). It still has 5 tuning gears and the original tailpiece. More photos available if requested. Bass is in Nashville. http://nashville.craigslist.org/msg/4055000591.html