fork stem nut advice needed.

need to replace the fork stem nut on my shovelhead trike but can't work out what the stem is off. I thought it was a FX/Sportster, bought the expensive nut to fit from dealer only to find that its not big enough, it needs to be bigger/wider. Does anyone know if a FL uses a larger nut? The nut i need is 24mm diameter with a pitch of 1.5.
Don't want the expense of ordering another stem nut only to be wrong
Or does anyone know where i can order a nut with those measurements?
Thanks.