R.C. & C.S. Clark
The ABC of the Clark brothers' stamp stands for Anti-Bilious Compound. Rollin C. Clark and Curtis S. Clark appear in the Cleveland city directory in 1871 as "chemists." Their stamps were first issued in August of 1873 and up to June 1, 1882. 275,000 were printed on silk paper and 32,297 on watermarked paper. The copy above is on silk paper.
A book of interest tables issued by the Clarks. It was not legal to include pictures of private die stamps on advertising pieces, though several companies did.