OP50 meaning


I’m looking for some background information on OP50, and although there are a lot of specifics about the strain and its origin, I cannot determine where the “OP” or “50” come from. This question was posted several years ago with no response and I cannot find this information in the literature. I’m only interested in anything definitive, not “could be” which is mostly what I’ve found online.

Thank you

This old gazette states that it is a mutant of E. coli B (Berkeley strain).

It seems the person who did the mutant screen should know, whoever that is.