To understand the X suffix to engine number DJ2050X you need to refer to the Service Bulletins, in this case number 85 of September 1949. The relevant section reads:
" ALL SINGLE CYLINDER MODELS.
PISTON AND OIL PUMPS. MODIFICATION.
All 1949 models incorporating the letter 'X' after the engine serial numbers, are fitted with a modified piston incorporating a scraper ring in the lower groove and a modified oil pump with a larger diameter (1/4") delivery plunger.
This arrangement has been introduced to eliminate piston slap which has been more prevalent since the use of post war materials was necessary. Reference should be made to the 1949 Single Cylinder Supplementary Spares list - Page 8 - for new part numbers relative."
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.