VERY good score there & I'm as jealous as hell. Congratulations.
The Swallow, first catalogued in 1937, was available as a B17 Swallow & a B15 Swallow.
Generally, the B17 & B15 series of saddles were basically the same, but with the B17 using the premium grade of leather while the B15 used the next level down. From 1937 there were four grades of leather.
The B17 Swallow used a wire thread to stitch the edges of the top & there was a section of leather that passed under the saddle about half way along. You can see the arrangement in the picture.
The B15 Swallow had two flaps that were an extention of the top that did the same thing. These met on the centreline & were held together by a copper or brass plate underneath & this was to prevent spreading of the curved top section.
According to Tony Colegrave, the noted Brooks historian & restorer, the wire stitching was the cause of much grief,however, the B15 Swallow did not use this & was much better, & in his view, the B15 is the better saddle of the two.
I'm not going to argue with him & yes, he is the man who repaired my B37 & who is restoring my Mansfield North Road.
Either one, in good serviceable condition will fetch the kind of money that would take your breath away, but both are subject to tearing in the area of the underneath flap & at the nose attachment point & while new tops can be made, it's folly to buy a damaged one for big bucks with an eye to having a new top made as both are based on a standard B17 chassis, & B17 Narrow saddles with ruined tops are worth very little, so this would be the more sensible way to get a chassis.
If anyone is contemplating having a Swallow made from a B17 chassis, then get the earlier one with the collar at the cantle plate where the under rails attach as these are of much better quality & were available, optionally, with chrome finish between about 1949 & something like 1957. These come up nice but hand made replica tops are not cheap.
Mid 50's B17 Swallow with optional Chrome chassis. Note method of attaching rails to cantle.
B15 Swallow. A thing of beauty.