Max,
The "SM" designation means that you have a particularly stubborn Mellotron that will occasionally need to be beaten and humiliated in order to work properly. No big deal. Once it knows that you're the BOSS, it will function just as indifferently as every other Mellotron.
Enjoy!
Actually, I think that you probably have a "Sound Sales" Mellotron. It may have an SMS 3 instead of an SMS 2 motor control card in it but other than that it is just about the same as most others.
Send it to me. I'll teach it to behave properly and send it back.
Frank Stickle (Mellotron Wrangler)