You're not the first to ask. The M is for their last name. The QH is for quarter horses. They were were horse breeders before they began raising and showing dogs. They still do the horses. The dogs I believe are more of a hobby. They hunt and show the dogs.
MQH Patterdale Terriers
I don't have their horse website. It's likely their last name and quarter horses. I believe they have something on their website on how they began having Patterdale Terriers.