Interesting question Leon, and one I don't know the answer to....we can easily assume from the article that at least some of the engine internals are non-original - but that is hardly surprising given the hard racing life of the machine!
A quick look at some old LDMCC machine registers has a 1926 TT Rep. Frame Number OF209 and Engine Number LH102 owned by a J.Cuyler. In a later machine register we have OF209 now with engine number IH102. Was this a typo? Easily made I guess - mixing up a "L" with an "I" ? (Edit - or the other way around)
Interesting Clew does not list an engine prefix with "IH" in his appendix, but does have "LH" for 4HP 1927 TT engine and the Douglas model identification on this forum has "IH" and no "LH" !!. So maybe the original designation of "LH" was incorrect and it was actually "IH"?? Don't know. Douglas appeared to have mixed up the sequence of their serial number prefixes around this time.
They don't appear to have liked doubling up of letters for serial numbers i.e. no "EE". But instead of going "ED" - miss "EE" then "EF" , they went "ED", "EH" ooops...."FH" , "GH", "LH" etc. (from Clew TBT).......(aside - I wonder if the apprentice stamping the numbers got a clip around the ear?! :-). Edit: Just joking )
But "IH" makes better sense in that sequence than "LH" ?? (Edit: i.e. no "HH")
Next OHV serial number was "EJ" for Reggie Pink 1927 Racing engine and from then on we appear to back into the sequence of "EJ", "EK" etc.
Not 100% sure but are the attached pictures of Reggie Pinks "EJ" version ?? (He had 3 OHV Douglas's from what I can ascertain - two 750's and a 500 - Edit: may have been 350cc, 750cc and 500cc - more research to do).
Pictures of Pink's 750 from;
https://www.douglasmotorcycles.net/index.php?topic=2490.msg9000#msg9000https://www.douglasmotorcycles.net/index.php?topic=2489.msg8984#msg8984cheers
Hutch