Significance of service number prefix 'letter'

Hi, I recall my father telling me that when he joined the RN in 1941 or 42 he was given a service number which began with an F, but when he later transfered to the army commandos the first letter changed to a P, the rest remained unchanged.
Is anyone able to verify this practice please?
I assume from this then that my father may have two service records?

My understanding is the Commando kept the service number from their original regiment - who also paid for them.
In my granddad's case he had two serial numbers, an Royal Marine and later an Army serial number, which you can see documented in his service records.

http://www.commandoveterans.org/cdoForum/posts/list/3563.page
http://www.commandoveterans.org/cdoForum/posts/list/3577.page

I assume from this then that my father may have two service records?

As in my case, you should apply to the Royal Navy and Army records.