I have gone through pretty much every MnDOT signing plans set since the very first one on I-35W in Bloomington in 1959 (SP 2782-37), and I have never seen a single plan sheet calling for any of these legislative numbers to appear on a sign. While it is distantly possible one or more of them might have appeared on temporary signing erected as part of a paving contract, I think it is far more likely that any such signing would have marked the new facility as a bypass or relocated mainline of the parallel US route, a very common strategy in the late 1950's/early 1960's.
Older plans sets do include the legislative route number in the highway designations on title sheets, plan sheet collars, etc. (i.e., not as part of the sign designs themselves), typically as something like "TH 35W=394," "TH 12=10," "TH 7=12," etc. That is not true for current plans sets and has not been since about the mid-1980's, though I haven't pinpointed the exact years this nomenclature fell into disuse.