Indeed, there is an 86 and an 86c filter thread standard. The first uses a 0.75 pitch, the "c" for "coarse" uses a 1mm thread.
Traditionally filters up to 39mm were 0.5mm pitch, and filters from 40mm ~ 82mm were 0.75 pitch (there are several exceptions).
Series Filter type lock rings usually used a courser thread (I'm not sure if it was metric or SAE, probably SAE)
Hoya lists their 86mm (and 95mm) filters with a 0.75mm pitch, B&W lists theirs with a 1mm pitch (86mm ~ 122mm).
The FE 50mm has a 1mm pitch. However, I have found that many filters with a 0.75mm pitch thread fine into the 1mm pitch lens thread - it's a function of the tolerance on the filter and the fact that you only thread it in a little over one complete turn. My Hoya filter threads in just fine with no binding (as do some others), but I have one (Toshiba I think) that starts binding before it bottoms. Ofcourse the B&W thread in fine. But what you really want to hear is that the Lee Filter adaptor ring threads in properly - unfortunately I can't tell you that (but I'm sure Lee can).
Coincidentally, I had a look at the Lee site, and they only list wide angle adaptor rings up to 82mm for the 100mm filters.