Thank you for your question. I am sorry about the confusion and I will be happy to help. The "TP" in the 561-TP-DST stands for "tract pack" which means that it comes with a set of three 561-MPU-DST's. The difference in price, which is a price per individual unit, is a discount because of the purchase of three units. There is no difference other than this.
I hope this helps!
Date published: 2015-12-21