NE2, I think you are referring to a different map than I am.
What you are saying seems to be based on the map posted above (and re-posted below). I tend to think it shows the US 77 alignment that existed up until 1955, but I really can't be sure. This map is rather ambiguous, and not highly accurate. Krum was not on SH 39.
As for the secondary jog west on the stairstep south of Lewisville (in the area of SH 121 BUS and FM 3040), that has always intrigued me. There was an old road west of SH 121 BUS in that area. It is visable on Historic Aerials, but has been wiped out by development. That could've been the main rd in the very early days.

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Here is that map that I was referring to concerning the stairstep between Lewisville and Garza (now Lake Dallas):

This one does appear to show US 77 going east for a hair out of Garza (Lake Dallas), before heading due south, then east into Lewisville.
My local history contact does not know of any other maps form this time period that would help on this debate.
Concerning the area between Lewisville and Lake Dallas:
I have always found it odd that there were two routes (US 77; and the county rd to the east that became FM 423 for it's last few years) between these tw o cities. Esp since Hickory Creek had to be crossed with major bridges. I have found no evidence that the eastern route (Former FM 423; Cowan/Main in Lk Dallas) was ever the primary route.
The weird thing is that the eastern route became FM 423 in 1948, which is the same year that construction started on the Lewisville Dam, which would close the route in the early-mid 50's. There was a vintage truss bridge over Hickory Creek on this route that had to be destroyed when the lake began to fill.