Guessing it's a quite scarce and very early Improved Model B (apparently the first Improved Model B was 14230) but you really want to ask the mods to move this topic to the 'Collectable' forum section as you will get a lot of help there.
Max; looking for: Baikal IZH 61 side lever rifle
Straight stock job, nice to see with duck bill cocking lever. Nice.
According to the John Knibbs book
Serial 14270 falls in the range of 14230 to14729 and is a Sixth batch BSA either standard or light pattern improved model B.
Manufactured Feb-Mar 1907 and left the factory between Feb 1907 and Nov 1908.
Photo looks like a standard pattern to me.
Ian