Proof I was in the Legacy Retirement System

I was enlisted in the USMC for 5 years from 2014-2019, and during the BRS eligibility period I did NOT opt-in (i.e. I stayed with the Legacy system)

The problem is, now I am commissioning in another branch and they are asking for proof that I was in the legacy system, and the USMC never put on my old LES or my OMPF anything about my retirement system. How can I prove I was not part of the BRS? If you switched to BRS you got all kinds of documentation, but from what I was told when I was enlisted if you kept the Legacy system your "proof" was basically that there wasn't any BRS documentation in your files, which my new branch is not satisfied with.

TLDR How does a USMC vet from 2014-2019 prove I was in the Legacy Retirement system for the entire duration of my enlistment?