Clincher is the camshaft bushes as a basic third indicator. First is the obvious matching case numbers stamped at the front. Look for unmolested and clear markings. OK if scuffed and dented a little, but consistent with age and style. Next see how the cases line up at the cylinder base, usual to find a small circular inspection stamp on the centre join. Should match perfectly, as should the sump plate face.
Next see if you have three good camshaft bushes, try the cam in each half, checking for ease of fit and wear on the bushes. Deep breath, assemble the cam into the cases, draw together, see if it turns. Good so far, nip up the fixings, cam should still turn easily, and you have saved a great deal of hassle and cash. Cam endfloat will be a good clunk, but this is of no consequence at this stage.
As KW says, unmatched casings can be machined to be used again but all engineering time and skill costs money and a set of rough looking but sound matched cases are a better option. Cracks can be welded, threads fixed at a lower cost than workshop engineering expertise.
Swarfy.
