@Ed Snible -- you're right about the catalogs after 1970, though many important sales (and less important) can be found scattered about (e.g., most NFA on Gallica or Archive, Glendining on Archive right up to the mid-90s, those below, others). Re: Gemini (Berk et al., 2005-2018): About 9 of 14 online as pdf? (They have great collection intro/bio material absent from lot archives; ACSearch has only partial Gemini coverage anyway.) Issuu: VI, VII, VIII, X, XI, XII, XIII, XIV (not IX), incl. the important Gemini XIII (w/ Chicago Art Institute's deaccession sale of Alexandrian, also Behnen's Decii). Archive: Gemini I only? The regular Berk MBS (early) & BBS catalogs from 1977 to present can be mostly or completely found on Archive (and/or HJB's website archive and/or Issuu for recent), including all 9 FPLs (not to be confused with "From the HJB Coin Cabinet" FPL which restarted the numbering c. 2014). (I've annotated 1-59 from those sources, though 42-43 had problem files; after 59 I've used hard copies, but think they're all online; they were on "newspaper paper" to the early 40s I think, so the copies aren't always great.) Similar w/ Victor England - CNG predecessors (as I first realized from Ed's gift for polydektes blog), though I think some of the MBS in the middle (c. MBS 60s - 80s maybe) aren't online. The CNA sales are great. Many coins reappear at CNG (and elsewhere) nowadays, often lacking the old CNA provenances. They're too early for (free) ACSearch so the pdf catalogs are all that's available online I think. I've got a lot of links to many other catalogs like that in a big file, so if anyone is looking for a link to a catalog, feel free to message me (here or elsewhere, email, etc.). (So far, I've only posted online my page of maybe 30 important Alexandrian catalogs of the 20th century, with links and annotations.) (Adding Issuu to my list above, mostly recent catalogs.)
If it's a British Celtic coin then it might be in the CCI and that can have provenances recorded. If you post a photo I can take a look (no promises I'll be able to find it though!)