I'd try googling for a shim to go from 26.8 to 27.2, two very common sizes in the 1970s and 80s road bike market. (26.8 - a lot of Japanese road bikes and of course the universal 27.2 for a lot of Reynolds tubing bikes. We are talking many thousands of each size being made so someone making that shim seems very likely. (Coke can shims are a very good substitute; my Peter Mooney has been using one for just that reason since 1995. (I had a choice between a SunTour MTB post, 26.8, for $30 and the same post in 27.2 with a better finish at $65. Money was tight at that time. That cheap two bolt post has been appreciated by my butt for 20+ years now. It has cost me a few bucks. I don't drink soft drinks so every few years I have to buy a Coke just to get the can! I change shims when I pull the post, but not to adjust height.)

Ben