The Skylark GS Supplement Service Manual lists the fuel filters as AC brand GF-94 for non Air Conditioned cars and AC brand GF-96 for cars with air conditioning. GF-96 filters occasionally show up on EBay but are a bit pricey. I have rarely seen a GF-94 on EBay.
However, GF-61 does show up quite often and are listed as for all cars having a 5/16" rubber hose fuel line. See photo attached. Note that the filter at the top is what you get when you order a GF-94 from a dealership. It does not fit into bracket and is not NOS.
Also, GF-423 shows up often and are listed for Buicks 1963-66 with air conditioning. The hose nipples are 5/16" This is what I have been using on my Air Conditioned GSs. Curiously, they come in either silver painted, red painted or that gold anodized finish. See photo attached.
I have also seen a GF-421 on EBay as listed for Oldsmobile with 3x2 carbs with air. Another number that shows up is GF-415. These also are for a 5/16" inlet and outlet hose lines. Can anyone tell me why these filters would not also work for a GS Nailhead? They are listed for big block cars from the 60s. They seem to work fine on my GSs, as far as I know. BTW, when they are installed into the correct bracket, you can not see the actual part number on the filter. Use AC fuel Filters as your search on EBay and see what comes up. Most sell for $25 to $45 plus shipping. Some are listed for much more $$$$.