That old part number is no longer used (we need to update that in the parts guide) It was a heavy duty nut - we just use normal nylock nuts - its a 5/16" x 18 thread pitch
Part number 8050331
Yes, please update the parts guide. Having the wrong part number for that nut cost my dealer two pins and nuts when he followed the guide last year and sold me the wrong ones (the nuts seized on the new pins which then had to be broken off to remove). Through trial and error we found #8050311
works. For those who aren't aware, it's the same nut used to secure the rudder blade to the rudder casting.