Photo: Barrel imprints on DLR pens : 1930's-40's*

'Onoto' was their brand name for a range of pens; 'Magna' was the name for a particular model (of which there were many versions).  All of these have the full company name: 'DE LA RUE & Co. LTD LONDON'. According to research carried out by other members of the Writing Equipment Society, the barrel stamp of Onoto's had changed to an abbreviated 'DLR LTD' by 1949/50.       

The lower right image is of a ball-point pen from a range called the DLR49, introduced 1949: imprint is 'THOMAS DE LA RUE & CO LTD LONDON': a throwback? 

In each case, as with other examples seen (whether chess, pens , books etc), either a full-company name is given, or a clearly identifiable abbreviation. In no case have I seen a vague stamp/imprint - such as the 'R' in the TRM.

It seems unlikely - to me - that DLR would have used such a mark as a simple 'R'.

* As I do not collect DLR pens, the images are extracted from photos in articles in the WES journal by various other members.