We got our hands on a whole sheet of some dark chocolate Soviet-made phenolic paper. The sheet did not have a factory stamp, but comes from the same pile of materials that were dated as early 1970s. Still, we can't make any difinitive claims, and are listing it as simply "vintage".
Please allow for some variation from block to block as the patina is not uniform throughout the sheet.
 
Approximately 5 x 1.5 x 0.7 inch per block.