I work as a sub-contractor for both the aerospace and automotive industries and, generally speaking, most company specifications state that drawings released for manufacture should be to a "letter" revision.
Pre-production drawings are normally given a "number" issue until the samples/prototypes/etc. have been cleared and accepted for manufacture.
However, most companies do not strictly adhere to this formula and may use alternative systems, especially on older drawings.
These inconsistencies should disappear, hopefully, as more manufacturers conform to international standards (e.g. ISO EN 9000, QS9000, etc.) which mandate that a formal revision control system be adopted.
In short, as long as you decide to use one method of revision control for your designs - and stick to it - you can't go far wrong.
Johnh