The 1926 Utah official (
https://images.archives.utah.gov/digital/collection/p17010coll68/id/23/rec/4) shows Spry as the end of the yet to be numbered highway.
The 1923 and 1927-32 Officials calls it Orton (no Spry shown at all) -
https://images.archives.utah.gov/digital/collection/p17010coll68/searchThe 1933 official goes back to showing Spry, with the 1937 Official returning to Orton.
Interestingly, the shape of the east end of UT 20 changes from a distinct dip downward to a straight across, suggesting that perhaps it met US 89 further south (road that does this used to exist - see frame 1 of the 1938 Garfield Co map:
https://catalog.archives.gov/id/5840167). However, the length of UT 20 never changes and the US 89 distance to Panguitch is always 10 (instead of 7 changing to 10).
UT 90 appears for the first time on the 1938 official and officials through 1941 show it as you have described.
Here's where it gets interesting. The 1945 official (next one available), shows UT 90 connecting US 91 (very near the Iron/Beaver line) all the way to US 89, never meeting UT 20, and ending at Spry Jct, 5 miles *north* of Orton (Google maps puts Spry here today), whereas earlier maps that did show Spry had it 3 miles *south* of Orton. The length of this UT 90 is not shown on the 1945 Official but it is on the 1947 Official and is 19 miles, matching the legislative definition (the route of previous UT 90 plus UT 20 to US 89 is shown as 21 miles). Reading "US 89"s blog post this matches up with legislative description of UT 90 and its changes.
The next one they have 1947, still shows a road to Spry (Exit 100 on I-15 to Dog Valley Rd at US 89?) but no longer labels it UT 90. By 1951, Utah Officials stopped showing both the road and Spry. Neither UT 90 alignment is shown as a state highway.
Pg. 141 of the 1944 Utah report (Table 10)
https://babel.hathitrust.org/cgi/pt?id=uc1.b3031660&view=1up&seq=141 shows UT 90 from US 91 to UT 20 removed from the state highway system at the same time the newer UT 90 route from US 91 to US 89 was added.
I was unable to find a similar table for around the time UT 20 would've been changed to prove the first UT 90 routing stayed out of the system until UT 20 moved to it.
This site -
https://onlineutah.us/spryhistory.shtml - says Spry was renamed a few times, including to Orton.