2/12/75 was the first board, and the next one after that was Orlando 8/31/71. Then, Dallas 3/4/75, Earls Court 5/24 and 5/25, St Louis 2/16/75, Houston 5/21/77, Vancouver 3/19/75, Dallas 3/5/75, Landover 5/28/77, etc.
Not sure how you are seeing a pattern there. They've been releasing boards mainly from Feb-March 1975, just as they have been for years, with some 1977 scattered in there. No surprise there. The 1971 Orlando board was a one-off, along with other shows that circulated online, like Southampton 1973.
They get the boards either from a former Showco employee, a person close to the band (like Ross Halfin), members/family of the band themselves (Jason Bonham), or from people who managed to get copies from the aforementioned. Unless you are one of those people, nobody knows what they still have.
Don't know why this is always such a mystery. It's not as if the tapes have been smuggled out of a bank vault.
Every time someone says "there are no more boards out there" another one shows up which proves everyone wrong. If there are any boards from January 1975, the only ones worth hearing would be the last night in Chicago (audience recording already exists), Detroit (another which the audience already exists for), or the unheard Pittsburgh show from 2/1. Hearing the tour opener, from 1/18, is probably the last thing that anyone really wants to hear, even though they might say they do. Plant's voice is trashed, Page has a broken finger, and who knows what shape the performance was in. I'd much rather have something from February or March instead. We already have audience recordings of Levee live, and you can see why the band dropped the number.
Give me Long Beach 3/12, San Diego 3/10, Philadelphia 2/8, Landover 2/10, Seattle 3/21, even Houston 2/27 or New York 2/3. Those are the remaining 1975 boards really worth hearing. Everything else is either too rough of voice, broken finger, or under-rehearsed. The LA shows are already covered by the Millard recordings.