Paul, you are missing the point ... I'm fully aware of the Wiki (and helped in its creation)..
This is a NATIONAL road, (white sign, black letters, no province prefix).  You have treated it a as a PROVINCIAL road (blue sign, white letters, with a prefix).
Very occasionally,  NATIONAL roads do have their responsibility changed to PROVINCIAL (& vice versa), but this is very rare. 
Google streetview does show the sign as recently as 6 months ago (https://www.google.com/maps/@16.6721203,99.2768462,3a,75y,201.91h,79.61t/data=!3m7!1e1!3m5!1sQMK9duKCAZ7H6DnPgMbx2Q!2e0!6shttps:%2F%2Fstreetviewpixels-pa.googleapis.com%2Fv1%2Fthumbnail%3Fcb_client%3Dmaps_sv.tactile%26w%3D900%26h%3D600%26pitch%3D10.385363918863035%26panoid%3DQMK9duKCAZ7H6DnPgMbx2Q%26yaw%3D201.905833071043!7i16384!8i8192?entry=ttu&g_ep=EgoyMDI1MDEyOC4wIKXMDSoASAFQAw%3D%3D).
I doubt if it has changed in the interim so to me, it should not have a province prefix added.
I trust this helps explain, but feel free to reply if not.
Russ