Underground Comix are known for using different prices, covers (art or color variations) and/or interior content to distinguish between printings.
1st print (April) 1973 has 0.50 USD cover price.
2nd print (1975) has 0.75 USD cover price.
3rd print (1977) and 4th print (1978) have 1.00 USD cover price.
This issue is a variant of
Dr. Atomic (Last Gasp, 1972 series) #2 [1st print 0.50 USD].
There exist further variants: