Hi John, It's deffo a "D", are there are faint markings on the tap cover plate, or as John says on the cocking arm?...Also I can't believe it has been refinished so hard in the distant past that all the stampings are gone, as this would involve a fair amount of metal removal, but it could be a possibility It was polished at one stage. If it was it was done well as the tap end of the barrel forging is still defined. Also check under the cocking lever close to the pivot for an array of inspection stamps, and a two or normally 3 digit number stamped in. This should correspond to the same number which is stamped on the end of the tap when removed.
If the inspection a tap serial numbers are there, then it is very strange as you would expect them to be polished away, and the block serial number is pretty unmolested.
Some other possible if long shot explanations;
Maybe it was returned to the factory for a variety of reasons that meant it needed a new action, and this was about the time they started the etched cylinders, so it came back with etching which is long gone
Or it was a D that had it's action swapped for an etched gun yonks ago...the patina certainly looks like it has all been together generations.
Or even maybe someone sneaked it out of the factory in bits
...Knibbsy will have the info on dispatch etc. at a price, which may be of help.
You could try rubbing it with paper and a crayon (like brass rubbing) on the cylinder to see if anything appears that you may not have noticed, or get it under a decent light and use a loupe in front of the loading hole, and on the cylinder.
All in all a bit of a mystery, I would love to see this up close and have a good look with a glass to do some detective work, are you going to the springer bash?