1921 was the last year for Morgans, so 1922 is a Peace dollar. It's worth about $27.
Mint state Morgans dated 1921 issued from the Denver Mint are very common with retail prices of $30.00-40.00
The US did not mint a dollar coin in 1968, therefore you don't have a 1968-D silver dollar.
Of all the Morgan dollars, the 1921 coins are the highest mintage most common. Regardless of mintmarks, values for circulated coins are $31.00-$40.00.
A 1978-D Eisenhower dollar has no silver in it and is only face value.
Sorry no O mint in 1921,it's a D or S. The 1921 date is so common even in mint state the buy value is below $40.00.
PCGS shows a retail price of about $1300, although that may be a bit on the high side.
These coins contain no silver and are worth only face value.
Please check your coin again. 1921-dated Peace dollars were only struck at Philadelphia. If your coin has a "D" above the "DO" in "DOLLAR" and the eagle's wings are outstretched, you have a Morgan dollar rather than a Peace dollar. There's more information at the question "What is the value of a 1921 Morgan silver dollar?"
In 1921 two different dollars were made both dated 1921. The most common is a Morgan dollar, some do have D or S mintmarks on the back of the coin just above the word DOLLAR. The other coin is a PEACE dollar that has this word on the back of the coin and will NOT have any mintmarks.
If it has 3 clearly repunched mintmark images, it will be worth about $30 in circulated condition.
There is no "series D silver dollar" (a coin) so I assume you're referring to a silver certificate. See the Related Question for more information.