When was 15 first signed in California? I know it was definitely in existence by 1964 as the California Highways and Public Works editions of that era referred to the northern segment as 15/91/466.
at the 15/40 junction this 1960 sign survives, with the 91 greened out, and the 15 moved over.
sign may indeed be older than 1960. photo is 1960.
somewhere I have more modern photos. the 66 and 40 were similarly covered up and moved over, respectively. that sign got replaced around 2008 to accommodate a retroreflective sign with an exit tab (your tax dollars at work!) but, as of last week, the 15 survives with a big patch covering the 91.
it is a shame because the I-40 shield was the rarely seen 30x27 size, which CA had as a standard from 1958-1962, for two full-width digits (I-40, as opposed to I-15, which was "one and a half" digits).
as far as I know, the only 30x27 shields which survive are at the MacArthur Maze for I-80.