It is a Monitor 1700 series. Parts and cartridges are readily available, just not at HD . Turn off the water, take all the parts, including the internal parts, with you to a big plumbing supply house.
Removing the cartrige retaining nut you have to be careful. If it is stuck and you just put a big wrench on it, you will cause FATAL DAMAGE to the valve body. Start soaking right now with white vinegar. Then after a day, start applying PB blaster and give that a day to work. Tap at it, even use a propane torch to heat it some. If these techniques do not work, come back for more advice. Just be careful not to twist that nut too hard!