J-1645 is one of the so-called "Goloid Metric Dollars" -- a series of pattern coins designed around a specific alloy patented by Dr. Wheeler W. Hubbell. The alloy derived its name from the small amount of gold mixed silver (Goloid) and the relationship (Metric) between the 90% gold/silver mixture and the remaining 10% copper. The Goloid Metric Dollars were smaller than the normal Silver Dollars of the period because of the addition of the gold.
If J-1645 looks familiar, it is because the head of Liberty used on this coin is identical to the head used on the $50 Union Patterns of 1877 (known as J-1546 and 1547). This head was also used in 1878 on J-1554, J-1554a, J-1554-b, J-1555, J-1556, and J-1556a, and in 1880 on J-1645, J-1646, and J-1647. Thus, J-1645 has a direct connection to one of the most famous and valuable of all U.S. Pattern coins.
The current population of J-1645 is estimated to be at least a dozen pieces, though someone needs to clean up auction appearances to identify different examples and to tie duplicate appearances together.
Gem examples are exceedingly rare, as most examples have been hairlined by past cleanings.