1970-D Washington Quarter Struck On Dime Stock.

Very interesting. Is the peripheral design weakness characteristic of these errors? It seems like that might happen with feed stock that is slightly too narrow.

Nice find Dearborn.
And great job listening to your inner self about the weakness to the peripheral design.

According to the data provide on the US mint site (https://www.usmint.gov/learn/coins-...Iq6y2cm7U5r) the specifications of a dime are:
Weight: 2.268 g, Dia 17.91 mm and thickness 1.35 mm
And of a quarter:
Weight 5.67 g, Dia 24.26 mm and thickness 1.75 mm

Thus a quarter struck on dime stock should weigh 2.268x(24.26/17.91)2 = 4.16 g.
It also should have a weak relief as the presses are adjusted to a thicker planchet.
The calculated weight correspond well with that of the coin of the OP which coin also has a rather weak relief.

I have used the diameter to calculate the weight and not the thickness as the specified thickness is that of the minted coin. This is the maximum and not the average thickness which should be used in the calculation. And of course the weight of a coin (cylinder) is proportional to the square of the diameter (assuming material and thickness are the same).

Likewise a dime struck on quarter stock should weigh 5.67x(17.91/24.26)2 = 3.09 g.
It should have a strong relief as the presses are adjusted to a thinner planchet.

A true pocket change treasure, that's the cherry on top! =)

Fantastic find!

Thanks everyone! I'll be putting this in my Dansco album for now..