MHH, who are you?

This makes me feel like a serious art collector. I have 8 peices of art all made by one particular artist. The problem is, I have no idea what his/her name is. I only know that their initials are MHH, and they did some of their best work in the 1970's....and they (or their surviving family) donates to a certain thrift store that I frequent.

Actually, the last two are unsigned, but I think I recognize his/her work.

I really want to know who this person is.