Dimpled chains were used in the 1950's and probably into the early 60's. I am no chain expert so can't say for sure when the dimples first appeared and when the practice ended. All I can safely say is that the dimples are a period correct indicator for the 1950's. I have had a few people tell me through the years that Duckworth had the dimples and Diamond did not. However, I have a very used Diamond chain that I have no idea what it came off of that has dimples, so, it would seem then that both the major motorcycle chains at the time came with dimples?
Below is a picture of the original chain that came off my '56 KHK. Is this the chain that left the factory with the bike? I don't know. It was worn out to the point where it may have seen every bit of the 35,000 miles that is on the bike?
The second little picture is of the Special Instructions section of a 1953 Season Order Blank. You apparently could request what chain make you preferred.
Back to the subject of the brake drum.
Strong, the picture you posted of the rear brake drum, does that picture go with the whole bike picture you posted? The brake drum picture lead me to believe that I was looking at an original unmolested bike since it had the dimpled chain with a fair amount of rust on it. ( right top corner ) You could tell in the picture that it has a chrome chain guard but it wasn't uncommon to replace a few items on a bike with chrome.
The bike picture looks like a restored bike to me. If it was original paint and a KHK it would have the KHK decal on the oil tank. As you mentioned it has an aftermarket exhaust system on it. My point is that if that is the bike that goes along with the brake drum picture it should not be used as an example of original. Restored machines are only as good as the interpretation of the restorer for originality and not necessarily how they left the factory. Especially if you don't know the history of the bike or the restorer. Factory photos and documents are the best way to go, along with pictures of original paint machines like Jim Garret's '54 KH. Even original paint bikes can certainly have been fiddled with but still serve as a pretty good indicator of how it was. There are plenty of very correct and beautiful restorations out there and plenty that are so incorrect it makes you wince. I should mention that in an earlier post I used my restored '53 KK as an example for the brake drum not having lugs. Even though I have spent the last 35 years making it as correct as possible through research it should also be excluded as an example of correctness to anyone that doesn't know me and my methods.
Sorry, didn't mean to get long winded.
Strong, in an earlier post on the brake drum subject you said that the lugs on the brake drum didn't come until 1957 along with the return of the 51 tooth sprocket. There must be something that makes you believe that. Right now without having any other proof it would make sense that in late 1956 they started transitioning to the 1957 set ups. My '56 is pretty late in the production year at Vin# 2268 and F6 frame date code. The frame has the indent for the Sportster rocker boxes. Until we find proof otherwise I'm going to believe 1956 was the first year for the lugs on the brake drum.