I could be wrong but I don’t believe the Legislative Route Numbers ever had anything pre-renumbering in their legislative description that dictated what they would be signed as. Daniel probably could elaborate more on what the legislative minutes used to say regarding the matter.
You aren't wrong; up to 1964 the only numbers that were legislatively "vetted" (more like "sign here" for routes submitted by the Division) were LRN's. SSR's were decided jointly by the various districts and/or HQ in Sacramento; the decision to number and what the number would be could be initiated at either level; as long as Sacramento signed off on the decision. That's one of the reasons why so many Valley connecting routes went unsigned until '64; the district offices didn't think overall navigation could be helped by signing certain routes -- and they didn't want to spend any additional manpower to post and maintain signage than was absolutely necessary. Of course AASHTO (and its single-A predecessor) had something to say regarding US highways -- but for SSR's, it was solely at the discretion of the Division of Highways.
Of course, since 1964 all state highways are legislatively designated (and relinquished!), regardless of shield type!
I'd be curious to see how something that plays in other states. It seems to me that having the flexibility for a local Caltrans district to decide on what a route would be signed as would be much more useful in a post-1964 numbering environment.
To that extent, relinquishments seem to be a direct result of the "shield = state maintenance" concept - cities want to maintain roads to urban standards (more bike lanes, decorative lighting, etc.) that don't always jive with CalTrans state highway standards, and most often (i.e. Route 160 between Freeport and Alkali Flat going through downtown Sacramento) this results in full shield removal.
But in comparison, US 101 along Van Ness Avenue in San Francisco has had a couple of years of work to provide new Bus Rapid Transit lanes and yet is definitively staying under state maintenance!
One example IMO where local routing decisions could potentially be better than the existing state-maintained/signed route is out in Woodland, where Route 113 uses the old Route 24/Alternate US 40 pathway towards Knights Landing, even though the parallel Yolo County Road 102 is newer, faster, and more direct.
Another thought is the 70/99 routings in the Feather River area: with 99 no longer being considered for bypasses of cities like Live Oak and Gridley, 70/149 has been the primary corridor for through traffic the last few years; logically that would all be one number from Natomas to Oroville rather than 3.
And then there's Richmond Parkway, locally acknowledged as the corridor for proposed Route 93, but not signed as such as it is not state maintained at this time - though currently kept up to a higher standard than say, another East Bay arterial in Route 185! It is a key connector between I-80 and I-580.
I always liked the example of Massachussetts, where route numbering signage is specifically not indicative of state maintenance at all (I've seen it mentioned on the forums how sections within towns have END STATE MAINTENANCE and BEGIN STATE MAINTENANCE signs).