Mount the fender before the front wheel.

Two 8mm 1,25mm pitch bolts on each side. The lower bigge hole (10mm 1,25mm pitch ) is for the front drum brake.

The front wheel:

I have not found any parts manual for the J model in pdf format, only from K and later, but this link on the web shows the J front wheel with the drum brake:

https://www.cmsnl.com/suzuki-gt380-1972-j-usa-e03_model16106/partslist/81501.html#.YvLRgXZByUl