The AC100 pictured below has no serial number plate and probably never had one. There are no tell-tale holes for the fixing screws. See the picture a little way below.

The circuit is that of the "100W Amplifier" - fixed bias but no brimistor. A serial number in the high 600s and manufacture in the autumn of 1965, would perhaps been the expectation given the general character of the amp and its chief component date codes. Chassis number is 01268. There are however at least a couple of later elements, both factory: a Daly capacitor with the date code "WK" = October, and an output transformer with solder tags on its underside. Such transformers are not found until the end of the year - serial numbers in the 1100s.

Click for a larger image. Serial number plates are normally centred above the centre screw hole on the lower edge of the board.

Two things seem possible: that the the amplifier was sent back to Erith early on for repair (though that would not explain the absence of serial number plate); or it was produced - or rather completed - under special circumstances, perhaps, for instance, in the days or weeks after the fire that devastated parts of the Vox Works in December 1965. It is hard to know for certain, however.