67 factory cuda tach

So if you take that gauge apart for any reason, you'll find those two contact studs are mounted in a small piece of phenolic board. Its simply attached by wire leads allowing it to drop through the case in either position. We'll assume that why the case wasn't marked.
'S' = Signal and for these tachs = Shorter stud whether you mark it before disassembly or not. The mo you know :)