<p>There were some M6 (hot shoe) spare parts made without serial numbers for replacement - on the M6J, for example. Leica also made spare top covers. Therefore, based on the OP's description this could be a parts camera; there are no assurances whatsoever of when the camera was made. There isn't even a seal on the lens mount on these cameras.</p>
<p>Because of these factors, there is really no assurance of the age of a camera based solely on the serial number or generation of the top plate unless you have the original box with matching serial number.</p>
<p>However, there is one sign that could indicate an older generation M6 chassis - if there is hard 'crinkling' paint around the back plate, then it's likely an early chassis (all others had smooth relatively soft semi-gloss black paint).</p>