30 sous is correct. 120 sous = $1; thus it's a quarter dollar note.
It was one of a wide variety of small denomination notes issued when an economic crisis affecting Britain, the US and of course Canada made silver coin unobtainable except at a hefty premium, so the little notes kept everyday commerce going by making it possible to change a bank note.
The paper was bank note paper of the time and it was a lot more durable than you might suppose - thin but tough. A lot of other small denomination notes of the period were printed on ordinary paper and ended up in tatters in short order.
The notes show a familiar coin of equivalent value, familiar to all, which helped to communicate the denomination at a time when illiteracy was prevalent. Ordinary folks did use these notes, certainly.
Two varieties exist, with (common) or without "Commr" at the lower right. Issued (fully signed) examples are known to exist of both varieties but they are rare. There were two sales in the past couple of months. They were clearly circulated notes.
The unceasing stream of unissued remainder notes on eBay comes from a very large hoard of uncut sheets. The notes were printed in sheets of 6, a set of the three denominations on the left and on the right. The note shown in this thread I believe to be original. The date is 1837, not 1857.
The three denominations were catalogued in the 1980 first edition of Charlton's catalogue on Canadian bank notes ($12 each in UNC but only $48 for a sheet of 6 - consider it a quantity discount). They will be included in a new book on merchant scrip which is now in preparation.