IMHO the minor differences between TOS and TAS vessels would justify differing designations - and internal differences could be more extreme, considering that only the TAS ships were seen with stasis weapons, and the interiors of the TOS ships in Romulan service differed quite a bit from the TAS Klingon interiors (too bad we never saw Klingon interiors in TOS).

The fact that we saw no definite ship in "Errand of Mercy" could mean that Klingons had partially working cloaks back then. Or then we could simply rejoice on the fact that for once we saw space combat at realistic distances, so that the opponents saw each other only as specks of light.

------------------
"One's ethics are determined by what we do when no one is looking" Nugget
Star Trek: Gamma Quadrant
Star Trek: Legacy
Read them, rate them, got money, film them

"...and I remain on the far side of crazy, I remain the mortal enemy of man, no hundred dollar cure will save me..." WoV

It would seem the effects people have never watched TOS, and are thinking that the only weapons ports on the ships are the opening at the bow and (on select ships) a corresponding hole at the stern. This is where torps and red (phaser?) beams always erupt from.

------------------
"The Internet. Is that thing still around?"
-Homer Simpson

The "Rules" ship used the red beam weapon firing from the bow opening, again suggesting that if a connection with TOS was attempted, it was a rather half-hearted attempt. Or then a deliberate effort to show that these old ships are useful only through major refitting that involves swapping the primary weapons.