This is a strange case of what is internally designated P-323 but apparently supposed to have S-323 shields
AFAICS, the functional classification has naff all to do with what shape shields the road gets, or even the number. Parts of US12 are P-14, part of MT3 is N-53 (ie national 53), parts of MT2 are 'off-system' (ie below secondary).
What we do have is, AFAICS:
1) signs in the field show it as MT323, rather than SR323 - and not just a mistake or two, but a lot of them suggesting purposefully doing this.
2) a policy of caring more about field signage of state highways than paper designations or 'what is supposed to be', so even if the signs are wrong, we'd have to view it as MT323 rather than SR323.
Therefore, I'm going to move what is currently SR323 in usamts to be MT323 in usamt. This shouldn't affect anyone other than me doing some housekeeping to enact this.