As I remember the one that was around our shop in the 1950's, it clipped onto the top if the valve spring, with a dial indicator that contacted the top of the rocker arm. Supposedly, it could be used on a running engine, but every time Dad tried to use it that way the dial indicator pointer would rotate around the dial at random due to the pointer slipping on the shaft. It could be used on a stopped engine by going through the same process of finding top dead center that was used for a feeler gauge. The only difference was that the feeler gauge was quicker, and more reliable. In my opinion, from seeing one actually in use in a top quality professional shop, the thing was a gimmick designed to separate a gullible mechanic from a big chunk of his money! If you manage to find the unicorn you're seeking, you'll probably love it!
A machinist with a modicum of common sense should be able to take the basic unit that's for sale on Ebay for $25.00 and adapt it to a specific valve train with a little bit of tinkering. The one that's priced at $200.00 has bunches of adapters, plus the thing that's really needed- - - - -a feeler gauge!
Jerry
