One-Tenth (1/10) Oz.
In the Spring of 2008, the surge in silver prices prompted the base valuation of the one-ounce silver pieces to increase to $50. This change led to additional denominations: a 2/5 ounce $20 piece, a 1/5 ounce $10 piece, and the 1/10 ounce $5 piece to accompany the $50 base one-ounce silver pieces.
The choice of increasing the one-ounce silver pieces to a $50 base in 2008 is an odd one. The silver price peaked at $20.92 on March 17th and dropped to $8.79 on October 28th, nowhere near $50. The new base could be attributed to factors other than the market price of silver at the time.
Like the other specie released at the same time, the naked back was stamped with a surprising number of hallmarks, many of which can be seen below.
There may be additional variations, if you have pictures please send to brian 'at' norfed 'dot' info and I will credit you here.