sbg2, yes. Thanks for setting me on the right track, should have figured it out myself...
It's not quite as simple as you're saying, because it has to be user freindly for a bunch of graphic artists who don't want to think about going to a separate table to create a new company and then back to the contacts etc., etc., but you got me going in the direction I needed.
Ted S, thanks for your explanation of significant numbers and their pitfalls. The numbers are significant, but in a way that will never change. Each has a letter code at the start to indicate client-number, job-number, ID-number etc., but the letter codes are universal, not specific to any other criterion.