Hi,
I have a Raleigh SPD brochure here from 1993, and it says that an MT5 (MMC frame) is 25.8mm, and all others (including MT3 and MT4) are 26.4mm.
Apologies, I'm not sure what twin-stay and mono-stay mean, but I'm presuming it means each seat stay continues to the lug on the back of the...