These are the Boxer cinderellas by the Polish master engraver Czeslaw Slania. There's 23 in the whole set so the last one, no. 23 featuring Cassius Clay, appears to be missing. The last two, Sonny Liston & Cassius Clay were engraved & printed later than the others, & in lesser numbers hence their greater price.
Jay Smith usually has good stocks of these, 1-21 go for $7-$15, while 22 &23 go for $35-$40 each.
My first set cost me about $100 which was par for the course here about 15 years ago, & then I spotted another set from Denmark at just under $17 postpaid to Australia which I just had to have at such a bargain price.
The main Slania site has 2 excellent pages devoted to these, worth having a quick look http://www.slaniastamps-heindorffhu...hletes01.htm