This item is the distinctive 0.600" Across Flats hex tappet guide that screws into the crankcase of all Douglas 2 3/4hp models (1912-1924).
An original tappet guide from a (circa 1917) engine was used as a template for replicating these guides - we CNC machine them from stainless steel to the original dimensions - so should be a direct replacement into your engine if yours are worn or missing.
These are CNC manufactured to the original dimensions in stainless steel. Price is each
Note the bore of these adjusters is 0.250" (as per the original), and we bore and ream this final dimension. While manufacturing these, I used a couple of examples as patterns (see accompanying photo of original spare engine - our new adjusters are on the left and the one on the left fitted, the original tappet guide is on the right) and noticed they did have some slight wear to the bore. Not sure how much difference the wear makes - but imagine they might leak more oil when worn.
As a final point - these tappet adjusters are screwed into the crankcase from outside - therefore if you are considering fitting new ones to an existing engine - it should be possible to just loosen the barrels enough to allow removal of tappets/adjusters, then unscrew the old tappet guides and screw in new ones, without the need to strip the timing case