AIUI the Beaufighters issued to the 4 USAAF nightfighter squadrons training in Britain between the end of March and the beginning of Aug 1943 were new. I've also seen evidence that 417th NFS received a batch of late production KW serialled Beaufighter VIf in late 1943 which would have been produced in the Spring of 1943. I suspect that that coincided with the replacement of AI.IV (arrowhead aerial on the nose) aircraft for AI.VIII (thimble radome) aircraft. For a long time after centimetric AI.VIII was introduced it was not permitted to be flown over enemy territory. So most of the Beaufighter VIf sent to North Africa after Operation Torch had the former and were replaced later. It was not finally cleared for use over enemy territory until April 1944
A few things to note.
Firstly, Beaufighter night fighter production ceased in January 1944 (production of the Coastal Command version continued until Sept 1945). Allow a few months for aircraft to be passed through an MU for final modifications and flown out to the Med and there were probably no new aircraft in the pipeline beyond mid-1944. So it becomes easy to see why earlier production aircraft, refurbished by the Civilian Repair Organisation in Britain, or handed over from RAF squadrons (see below) would have become necessary as 1944 wore on.
Mosquito night fighters were in short supply in 1943/44. The last 5 Beaufighter night fighter units in Britain didn't convert onto Mosquitos until Feb-July 1944. Add to that the need to replace early NF.II, AI.IV equipped, with later marks with AI.VIII and from 1944 AI.X, compounds the problem. Production always lagged behind demand. Initially there were hopes that the USAAF might be allocated some in the first half of 1944 but those could not be fulfilled. Ultimately until late 1944 all USAAF requests for Mosquito night fighters for service in either NWE or the Med had to be turned down (see below).
But the USAAF compounded its night fighter problems in 1944 by cancelling all its P-61 commitments for the Med at the beginning of Feb 1944. Given the lack of available Mosquitos this had to be reversed later in the year. But given low P-61 production levels from Aug 1943 (200 P-61A and 450 P-61B to July 1945) it would have meant shifting priority for allocations from the Pacific, where there was also a need, to the Med.
When the 4 USAAF night fighter squadrons arrived in the Mediterranean in Jul/Aug1943 there were 7 Beaufighter NF squadrons equipped with the same Beaufighter VIf as the USAAF squadrons around the Med. 89 moved to Ceylon in Oct 1943, 219 returned to Britain in Jan 1944 to re-equip with Mosquitos, 153 disbanded in Sept 1944, 46 returned to Britain in Dec 1944 becoming a transport squadron and 108 disbanded in March 1945. The remaining pair, 600 and 255, re-equipped with Mosquito XIX between Dec 1944 and Feb 1945, with the latter receiving some NF.30 in April.
The only NF/intruder Mosquito squadrons in the Med in 1943/44 were 23 (with radarless intruder Mk.II & later VI between Dec 1942 and May 1944 specialising in intruder work) and 256. The latter sent a detachment to Malta in July/Aug 1943 with centimetric AI.VIII equipped Mosquito XII to cover the landings in Sicily and the whole squadron returned to Malta in Oct. It remained in the Med until after the end of the war. A couple of the other Beaufighter squadrons were lucky enough to acquire a few Mosquitos for short periods in 1944. And until Dec 1944 that is it NF Mosquito wise.
The USAAF request for Mosquito night fighters was finally met in part in Dec 1944. That month the 416th NFS received 12 Mosquito XIX and 7 NF.30. Note that is the same month the 2 remaining RAF squadrons converted and that it received the NF.30 variant before either of the 2 RAF squadrons.
Of the remaining 3 USAAF Beaufighter squadrons in the Med / southern France area, the 414th NFS began to receive P-61 in Dec 1944 followed by the 415th & 417th in March 1945.
One final note. In Aug 1944 the 427th NFS arrived in the Med equipped with P-61s. Initially it was intended that it should go to southern Russia to defend the Russian airfields being used for the shuttle bombing of Germany. That plan was cancelled and after 2-3 weeks in Italy in Sept they proceeded east to the CBI theatre.