The RAOC 'Chilwell Cards' for C3398 and C3360 simply state that they were 'ex-RASC' and give no frame number info. It's much easier to piece together the details of the RAOC contract machines such as Nortons and most BSAs.
The Triumphs were generally delivered to Feltham and it's clear that they were mixed up before the WD serial numbers were applied. They were issued the catalogue refs 19 and 14, despite being earlier than C5108 (10) and C6128 (6) which indicates how chaotic things were.
If you use a pre-war WD serial / census number from the early contracts on an OHV machine, it will be a little incongruous, whereas the earlier frame would not stand out to most eyes on a 3HW....Might it not be better to mark it in line with the engine number, or indeed to give it a 'rebuild' series number.