Is this a good deal for the MP purple shaft 284 528 solid lifter cam and lifters new in box.

mopowers said
Probably a stupid question, but if those are solid lifters, why do they have a lock ring?

Lots of solids have a plunger similar to a hydraulic lifter. They are oil metering solid lifters. They don't pump up, but the plunger controls the oil flow. They are arguably superior to a standard solid lifter.