Allow letters to the Lookup # field

I think it would be nice to allow letters to be put into the lookup # field on the "create file" screen. Instead it has to be numbers only. But over on "Miscellaneous Changes" screen it will allow you to put letters in your lookup numbers. We have our files with the file number and then the client's initials or name, i.e. 240020_MCK or 230127_Patel. It will NOT allow me to put letters in the field on the create new file screen but it will on Miscellaneous Changes.