Sovcit trying to pay their mortgage.

Nope lol. According to Wikipedia its nickle and copper:

Its current version is composed of two layers of cupronickel (75% copper, 25% nickel) clad on a core of pure copper.[2] With the cupronickel layers comprising 1/3 of total weight, the coin's overall composition is therefore 8.33% nickel, 91.67% copper. Its weight is 0.1823 troy oz. or 0.2000 avoirdupois oz. (5.670 grams).

Official US Mint description:
https://www.usmint.gov/learn/coins-and-medals/circulating-coins/quarter

Quarters minted before 1964 are indeed 90% silver. However, the weight is way off from sovcit's testimony.
Edit: A word
To be fair, it could be a pre-1965 quarter. Those had silver in them. I don't think .9 oz, but maybe one year?
- A silver dollar contains 0.9oz silver.
  A silver quarter is 0.225oz.
  
  There you go, they're not even right if it's a silver quarter. Thanks.
- Nah, these coins are minted for collection and investment purposes and are not meant to use as direct payment (although you can because they hold a nominal value). It doesn't make sense for any government to use material that is 100 times more expensive than the value they want to attach to it.
  Sovcit just payed of $0.25 of their debt with a $28 coin because they are incredibly smart.
  
  That's exactly why they stopped minting silver coins in the 1960s though. Because they started being worth more in silver than their value. Still true about pennies, I think.
  
  There were actual coins made out of silver that are not considered bullion coins like Morgan Silver Dollar or the Peace Silver Dollar. Those were made for everyday use and they contained around 0.75 troy ounces of pure silver.
  This leads me to believe that the 0.9 troy ounce coin is some sort of special minting because that would be a pretty heavy coin for such a low value (around 28g). But maybe times were just different back when the gold standard was still in effect, idk.
  I'll have to admit that I base my thought process almost entirely on my recent discovery that I actually possess a valuable coin and the bit of background reading after that discovery.
  
  I'm pretty sure that the quarters weren't that pure, I don't know. I do know people were melting them down (or at least were suspected of melting them down, I don't know if they actually were), so they changed the composition. My grandfather collected coins and I sort of vaguely remember all of this from him.
  I do have a ton of silver dimes and quarters in a drawer though. I should probably just sell them because I was keeping them around as a 'just in case' thing but the value of silver is not exactly shooting up and I really don't care about coin collecting beyond the design of coins in various countries being interesting from an artistic sense.

Quarters minted before 1964 are 90% silver, but even then the total weight is 0.1808 troy ounces per quarter. A quarter would have to be over five times as large in order to be composed of .9 oz of silver.

Now if sovcit wanted to do the bank right(!) here, he'd attach a .999 troy oz silver coin, such as a US Silver Eagle, which would set sovcit back about $40 or so. Postage would increase, if sovcits pay postage, because the envelope would need to be padded and the increased weight of the coin versus the quarter would increase rates. I did not do the math on that - someone else can.

No, no, you gotta think sovict.

Its a coin, that is silver in color, i.e a "silver coin," that is .9 0z.

No, what the sovcit attached is called a bullion coin. They usually have a nominal value, so they are official legal tender and you can pay with them in a store, but their actual worth is basically the price of the precious metal they are minted from (+collector's value for some coins) which is usually much higher. 0.9 troy ounces of pure silver are around $28.

It is the coin holder's duty to liquidate the coin before payment, so the sovcit basically just scammed themselves and gave their coin away for the nominal value (although it will likely be returned as this letter will not be accepted for obvious reasons).

That’s not a bullion coin, it’s just a pre-64 quarter. It’s about $20 in silver, depending on spot price.
I collect coins, not silver, and these are not special in any way. I sell them for melt value to buy better coins.