Vintage Sun Super Tach. Why are they worth?

I've been looking at vintage sun tach's also, crazy prices. I still have a super tach II but they don't appeal to me like the original sun super tach. I found a nice one at a swap meet last year for $300.00 and passed on it, I should have bought it. They started making nice new one's by sunpro but they sold out to bosh and they quit putting the sun logo on them, I'd like to even find one of the newer ones. I've been thinking about finding an old super tach with bad chrome and putting the internals in a new bosh cup.