The logo in the upper-left corner is that of the Chiang Mai Provincial Administrative Organization, which probably means the road is under the PAO’s authority. I’m not quite sure why the ref number follows the DRR’s format instead of ชม.ถ1-xxxx, but there are several others listed here.
Yes, that’s the road ref.
And to show one of the finest examples of the difficulties of mapping in Thailand, on my doorstep (by the new Chiang Mai Exhibition & Convention Centre) we have the CM.4307
At the northern end, in the monastery, we find the sign … http://1drv.ms/1gbwSBs
Its on a single track, poorly paved road.
But, head south, and it finishes as an 2 x 4 lane dual carriageway … All in the space of 1.09 km !
This helps explain the discrepancies between road authority and ref number. It appears these roads were originally built by the Office of Accelerated Rural Development (รพช., with the Singh-inside-a-gear symbol). The Office’s authority was later transferred to the Department of Rural Roads, and the DRR subsequently ceded the road to the Chiang Mai PAO. The ref number was given by the ARD and appears to still be used by the PAO, but is not recognised by the DRR.
It seems that for all roads below National Highway the numbers are mere job numbers. We can put them in the map or not, it makes no difference. There are no signposts showing them and I would never try to ask a local “Is this road number xxxx?”. For an example in Germany of a road that goes from Residential to 4-lane trunk there is an example in the German wiki. But yes, when I see such a sign I do map the ref number…