Its a modification of the Yak-9D series. The modification consisted of replacing the rear cockpit with a bay that could house 4 bomb bays, placed in pairs. One behind the other. In each could be suspended four FAB-100 (220lb) bombs. Alternatively four clusters of PTAB 2kg anti tank bombs could be carried. The gun armament was the same as the Yak 9D. A series of production batches of around 109 aircraft where built and delivered to the 130th Fighter Division command by F Shinkarenko. Operational trials where to prove unsatisfactory due to the lack of a special bomb site and bad handling qualities when loaded with bombs.
This from 'Yakovlev Aircraft' by Bill Gunston and Yefim Gordon.
Yak-9B. Known to the Yakovlev OKB as the Yak-9L, the prototype of this fighter bomber was modified Yak-9D No.14-20. The only change was to equip the space behind the seat with either 4 tubes for FAB-100 bombs' carried nose-up at an angle of 75deg, or 4 cannisters each for 32PTAB-2.5-1.5(1.5kg) bomblets. Normal load was 2 bombs or containers only. Bombs were loaded by putting the tailwheel on a trestle, removing the detachable rear canopy fairing and attaching a portable winch. Only very experienced pilots were permitted to carry the maximum load. The prototype was converted by 20 March 1944, but service trials did not begin until 18 December 1944. Eventually production was limited to 109 aircraft, partly because of lack of accurate sighting (though in a 45deg dive an error within 50m was usual) and because of tricky handling with maximum load and full tanks.
Found this on another forum and thought I would share with yall. Enjoy!