A 1947 ROF No.5 would have a BK serial number prefix. Except for the really late production and condition, that is nothing special. Many carbines were set aside as less than desireable, for a lot of unsubstantiated reasons.
As for a spigot launcher, I'd be wary. The rod depends on gas pressure in the bore to push the weight of the grenade downrange. The troops were issued special high-power blank cartridges for that, and the physics were well understood with full-length No.4 barrels. The No.5 is shorter, so the pressures won't be the same, and the range even less. Unless, your grenade launcher is marked and well described in the literature as unique to the No.5, I'd think twice about swallowing any clever advertising copy about rare, vintage, experimental or special. The two could be completely unrelated and just happen to be together in your hands.