As has no doubt been mentioned previously, marine offices were issued batches of books with pre printed numbers to be issued as required to new entrants to the MN.
Naturally the busier offices got through their stocks quicker than the quieter ones, hence two men issued a book on the same day at different offices could have wildly different numbers, indeed a man issued a book a year after someone else could easily have a lower number if it was issued at one of the less busy offices.
These days that doesn't happen as there's a central database kept in Cardiff, so regardless of whether a book is issued at an MCA marine office or sent from Cardiff it's issued with the next available number.
Dates with regard the changeover from R numbers to UK were somewhat variable. UK numbers were issued from (I think) 1971, however R numbers were still being issued some time afterwards as offices used up stocks of books.
As an example; about 15 years ago I was signing off a Motorman and did a double take when I clocked on the articles that his discharge book had an R number, this being a bloke who at the time would have only been about 40. I checked his book and sure enough he had an R number. I queried this with him and he explained that when he first went to sea in the mid 80s he had his discharge book issued at his local office which was Hartlepool. That office must have still had a stock of the older books - and obviously didn't issue many - hence he was issued with a number in the R9XXXXX series, even though most other offices started issuing UK numbers nearly 15 years earlier.
