I can't make the eBay link work...I've seen this RAF parts list in the past. It seems to be a blue cover version of the generic early-war parts list and not contract-specific. Later lists were unillustrated but tended to quote engine and frame numbers.
There are still a lot of unknown gaps in the RAF contract engine and frame numbers.
In short, because that was the Air Ministry. Although a very new service, they guarded their purchasing power and privileges jealously, until forced to accept more Ministry of Supply involvement.
It was standard in the contract terms that 'x' amount of literature be supplied and it seems to have been the habit where possible to reference contracts, however, there were plain replacements in existence too.
As time went on, the RAF machines did vary....the sidecar outfits gained 18" wheels etc.
You might have noticed that NOS spare parts are often offered as RAF..This is because the RAF had their own numbering system and it was more prominent on the labelling that the War Office identification !
There are no frame or engine numbers in the RAF Norton spares book or any contract ink stamps,the only difference is on page 2 under note: PART No. 3605 K.S. CRANK RUBBER IS NOT FITTEDTO R.A.F. MODELS,the nearest other book to it is CONTRACT 294/C/5612 which has the same part numbers,prices & illustrations,the book probably dates from 1939/40.