Were they simply secret routes -- unsigned but state-maintained roads with high numbers, as some other states have? Probably not. As I'll show later, these 500-series routes appeared to be publicly signed.
The map shown here, titled Essex County, was published by the county Associated Boards of Trade in 1926. The mapmakers were also kind enough to include a short log of the area's highways, and a list of highways found in each town.
Very few maps have included a highway log (summary of routing, intersections, and length for all highways). This log supports the notion that the 500-series routes were signposted and used for navigation.
Another nice touch: listing the highways in each town, along with points of interest and other info. Again, the 500-series routes appear as legitimate as routes 107 and 129. Notice the improvised US shield for route 1A.
Salem, scanned from 1928 Green Book, by Matt Steffora
Lynn, scanned from 1928 Green Book, by Matt Steffora
I haven't found any other info on this. Dan "SPUI" Moraseski says "C" routes are city routes, and believes there were two separate 500 systems -- the 1920s group and the later civil defense routes. The civil defense 500 series were more accurately a "plus 500" series; if the main route was 100 or above (such as MA 128), the parallel route would be out of the 500s (such as 628).