Go shoot it. If the Kadet slide fits the frame snugly and the gun operates OK, then I wouldn't worry about the fit. My bulllseye match gun is a Kadet kit put on a 75B frame with a firing pin block lifter still in place. My understanding is that the frames that come on the complete Kadet pistol are just 75B frames and parts, less the firing pin block lifter lever. Even if the Kadet adapter isn't snug in the frame, it isn't going to affect the accuracy much since the barrel is pinned to the Kadet adapter anyway so the sight versus barrel relationship stays constant, unlike a centerfire gun, where the loose fit means the barrel isn't snug.
To me there is no reason to be concerned that the Kadet kit was added to a 75B frame, as long as both are otherwise in good shape. I wouldn't even worry about the serial numbers being different between an adapter and a barrel, as long as the barrel was snug.
Joe