I'm not exactly sure what 'AS' stands for but it is clear from the few remaining factory ledgers that it indicated a spare engine. It was used as a prefix on spare Big 4 motors too. Strangely spare frames had the usual 'W' prefix.
It's a bit of a puzzle where these high numbers came from. Spare engines and frames pre-war were just part of the usual (non-matched) sequence. Maybe someone at Nortons thought that the war would be over by Christmas and guessed they'd never make more than 15000 WD bikes ?
There was a batch of AS15000 - AS16000 odd numbers issued in October / November 1940, supplied to Chilwell and then a series of spare engines for the RAF begining at AS18000. There are lots of gaps and AS20104 was delivered to the RAF on 25/11/40.
I've seen several spare part number plates but until now they've all been oval with a 'Cat No.' shown