Kev,
Yes the plug you have circled is the fill hole. The transmission in my 1941 and 42 trucks is the same as yours for the most part. I would recommend that you fill the case to about 1/2 below the fill plug hole or else when the oil expands or you park on a hill facing downward, it will puke oil out of the front shaft housing and potentially on your clutch.
I think we all have chased the safe for "yellow metal" issue rabbit at one time. It can be confusing. None of the additives are necessary in these old gearboxes. I also use the straight 90 mineral oil "tractor fluid" from Tractor Supply that Mike mentioned in all of the differential, transfer case, and transmission cases.
Welcome to the site, and enjoy your truck! It would be great to share some pictures.
Paul
