Bingo on the fine threads! 5/16-24. Luckily my local Ace hardware carries them. I would have taken the one I had to the store to match it, but after trying for a half hour to get it on the stud and engaged with one finger, I just didn't have the will to take it off.
Thanks for your help everyone!
BTW, if you need a fuel line from the pump to the filters on the Lehman, Bomac can make one up. AD can't get them at the present time.