They are found on 1865 Newspaper stamps PR2b, 3b 4b My listing only shows unused with about a 15% premium EDITED: Also on 1847 St Louis provisionals - very valuable Charleston 1849 Carrier Some Confederate stamps; Memphis, Victoria Onionskin - "Pelure Paper" from French peel
Another US experimental paper was blue rag paper used in the 1908 series for all denominations except the 50c & $1. Only some 4000 each were printed of the 3c - 15c making them quite rare.
USA UNITED STATES 2c REVENUE BANK CHECK PERFORATED ESSAY ONION SKIN PAPER (NVR8 ( 361406470547 ) is on ebay...Just wondering about this one
Found a pelure (onionskin) Russian stamp in an old album,,,,