I will share my client numbering system that would help with this latest kerfuffle. When I do a clients first return, their number is the current tax year being done, then a 3 digit number that automatically increases by 1 with each new client. This allows me a nice sort and tells me at a glance how long the client has been with me. I purchase a tax client list in 1980 with clients going back to 1966.

My longest running client would be 1966007 and my newest client would be 2021001.