I'm probably the only one who read the topic and wondered what in the world LRNs could possibly have to do with US-101.
The really weird thing is how many acronyms and abbreviations are really generally just accepted as part of California Highway speak. Here is a general list of what I've observed used over the years:
LRN: Legislative Route Number; these are generally referenced to unsigned state highways prior to the 1964 renumbering or highways that were never signed afterwards.
SSR: Signed State Route; generally referenced to signed state highways prior to the 1964 renumbering.
CA: Pretty much just the two letter designation everyone uses for a state highway but for some reason this seems to often refer to post-1964 signed state highways.
CASR: California State Route; for some reason this popular also and seemed to be a combination of SSR and CA.
SCR: Signed Couty Route; basically referring to the letter designated signed county highways.
Generally when I'm writing a road blog or some sort of historical thread on something in California I'll usually refer to full name of what I'm talking about following by the acronym from then on. Really this is the only state I've lived in where this just seems to be part of the road geek culture. Most other states I've lived in you generally hear what you expect; State, County, US, Interstate, etc. I suppose there are some oddities like Trunk-Lines in Michigan or State Road in Florida.