Why are pat pending bricks rarer and treasured more than patented bricks?
As I understand it, "Pat. Pend." bricks were made in the 1960's.

Later around 1970 they removed "Pat. Pend." by carving it out of the molds, so there is a lump in place of it. These are sometimes called "Pat. Pend. removed". Those ran until 1976-ish?

I don't think any of these bricks are much more valuable than the others. Old generic bricks aren't generally worth very much. If one wanted to have genuine 1970's pieces for personal reasons, I guess they might look for these markings.

I don't think all pieces had those markings though.