I remember seeing a chart for the different types of neon indicator lamps and their type numbers (like NE-1, NE-51, etc) but I have lost it. Does anyone know of a comprehensive reference for these types of lamps?

Good question. I have never found a comprehensive chart that lists all types, so have compiled my own based on a combination of multiple different references.  I hope the attached file may be useful.  If you are able to fill in any of the gaps that would also be appreciated!  It's been a very long time since I made this sheet and perhaps in the mean time other references may be available, and enable an update.

Thank you!
Aha! I found the document that I was talking about! I hope this will be of help.