Petter PJ3W in 1970's Narrowboat Any age and info appreciated

[Geofiz]

Hi all,
New to the Forum, so forgive me if filed in the wrong place.
I recently acquired and old Les Allen Narrowboat with a Petter PJ3W Engine and Velvet Drive box. The serial No. of the engine is 2124WM. I am assuming that the 'M' stands for Marinised. Correct me if I am wrong.  I am struggling to find a date for the engine other than the Product date range of 1962 to 1985.  Given the relatively low serial No, I am assuming that its closer to the start date, given the popularity of these engines. This one is running quite well though there is a small wheeze on w one cylinder. I have yet to investigate as yet as still getting the boat sorted out first!
Would greatly appreciate any specific information on this engine that anyone may have please as it would be notice to add it to the Narrowboat's own historic past.

Kindest regards
Graham