The steering wheel center button on a 914 is the same that is used on the same vintage 911 / 912, right? I should know this, but for some reason I am not 100% sure.

Depends on what you mean by "vintage". In the years 1970-1973, the horn buttons ("pads") should be swappable within the model year. In 1974, the 911 steering wheel is significantly different, so the interchange stops. Pre-1970 911 horn pads, while different looking, should fit '70-'72 914s. Mark Britell has one on his car. The Cap'n