It is estimated that there are less than 40 instances of this finely struck matte proof, which was exclusively available in 1964 Special Mint Sets.
1964 SMS: $8,750 in MS65
This kind of inaccuracy was made feasible by the fact that mintmarks were mostly still hand-punched onto functioning dies during the 20th century.
1942-D (D Over Horizontal D): $4,620 in MS65
The word "MONTICELLO" and other inscriptions (seen above) are the objects that have the double effect; the structure itself is not.
1939-P (Double Monticello): $1,500 in MS65
It's interesting to note that this repunched mintmark coin has an initially struck "D" that is upside-down!
1946-D (D Over Inverted D): $1,350 in MS65
Nevertheless, an error occurred with about 1,655 proofs that did not have the "S" mintmark.
1971-S (No S): $780 in PR65
This is an additional 1943-P 35% silver nickel variant. One type of what's commonly referred to as an overdate mistake is the "3 Over 2" or "3/2" variant.