That sort of thing is really common in Oz due to the requirement for IFR aircraft in CTA to be equipped with an assigned altitude indicator or altitude alerter. I made one for a Navajo I manage here in the US. I felt like I was missing something after the Oz fleet & then flying turbine equipment with altitude selectors.
I used an electro-mechanical digit selector I bought from an electronics surplus shop. They had hundreds of them salvaged from various devices. The type I used can have as many or few digits as you like - just add or subtract digit 'units' until you have what you want. 5 digits, in my case. It uses a + push button above each digit to increment, and a - push button below each digit to decrement. Other types, as a previous poster wrote, can use a thumbwheel instead of push buttons but the effect is the same.
I mounted it in a spare 3" instrument cutout after removing the electrical contacts from its rear. Probably didn't need to remove the contact spades but I didn't want to chance anything catching or rubbing on them. My mounting also includes a cradle to face it up towards me a bit. Makes it easier to use & read.
You could use two. A 3 digit one for heading, & 4 or 5 digits for altitude. They should both fit in the same 3" cutout if you have an unused one.
Last edited by Tinstaafl; 6th Jul 2013 at 18:03.