There are two different fine spline birfields. The 75 through 78 and 79 thru 90. The first generation was a little shorter and the flats
were ground deeper on each side. The was shortened , left more material at the flats ( stronger shell ) and changed the heat treat process.
You can either run without the clip, or switch to Aisin style hubs like those found on 60's or early live axle Toyota 4x4's ( pre-85 ). Prior to 79
the 40 came from the factory full time 4x4 with drive plates rather than locking hubs. Since locking hubs weren't offered on the 40 from Toyota until the birfield change the Aisin hubss were built to take the shorter birfield. Warn, Selectro and other aftermarket hub builders were building hubs for the 40 since it's intro to the USA. They built for the early coarse spline and the taller hub to fit what was offered.
There are tall Aisin hubs floating around. The FJ55 was offered with hubs a year or two before the 40. The 76~78 FJ55 had basically the same disc brake front end as the FJ40 but with locking hubs. The Aisins on that application were taller than those found on the 40,60 and mini trucks