1 silver kopek (aka "fish scale") of tzar Petr Alexeevitch (aka Peter The Great), the part of the dye with the year of coinage is missing (should be below the horse in a middle) - this happened very often since the blank for the coin was smaller than the dye itself. This particular coin was issued from 1696 till 1717 and had different weight depending on the year of the coinage - 1696 and 1697 weighted 0.4 grams, 1698 had both 0.4 and 0.28 grams and 1699-1717 were 0.28 grams or a bit less. In my personal experience, coins from the latest years can can be as low as 0.19 grams.
If you read some Russian (or can live with an awkward Google translation), check this:
http://silver-copeck.ru/215.html , at the bottom of the page there are the links by the year to an online catalog of almost all known variants of this coin.