Back in the dark ages, most legacy customs pilots were GS-12s. There were only limited -13 slots, reserved for PICs in multi-tubine A/C (-60, citation, etc). I think it was about a 1:4 ratio and the 13 slots were non-competitive “merit based”. The ultimate good ol’ boy network.
I’m not exactly sure when the practice ended and everyone became a -13 but it was well before the merger, ~late 90s. Well before my time. The P-3 PICs getting GS-14 is another story, but even that is no longer the case. New pilots in P-3 branches will no longer get a 14.