Joerg - Is it possible that the original gauge wasn't working, which required the add-on gauge so you could keep track of oil pressure? My Grandpa's truck had something similar, where he left the old non-working gauge in the dash assembly and just added another to do the job. It was done back in the early 70's IIRC, when he replaced the original 216 with a 235 car engine. He didn't care about it looking original, just functional.
Bill Hanlon asked a good question on what engine you're running as that may affect what gauge you use. Can you let us know what engine you have and if you know why the add on gauge is there???
Once we know a bit more on these details folks can offer more/better suggestions.
