It's on the notes themselves, really. and it wasn't just the 5's in the 50's. It all amounts to what the notes could be used for. While most stores, people and banks had no problem in accepting them, there were some specific rules that were in place on some of the notes at certain times. You could certainly buy your groceries, pay your bills, go to the hardware store, etc. and pay with any of the notes, so most people were not affected. But there were some limitations that were in place for things such as Duties on Imports and Interest on Public Debt. These restrictions were in place because the different types of banknotes were issued with separate funds. For instance one type of banknote called the "United States Note" (AKA Legal Tender or Red Seal Notes) were restricted for being used to repay the public debt with notes that are a part of the public debt. This was because the way that the country funded (or at any rate accounted) for the notes, they were considered a direct obligation of the United States, and thus were public debt. At the time, it was seen as fiscally irresponsible to repay debt with notes that were issued that were a large part of that debt.
After a while, the Legal Tender Notes lessened their restrictions and began using the generic phrase "Legal Tender For All Debts Public and Private", which in effect made them indistinguishable from Federal Reserve Notes, and they were eventually discontinued.
Another interesting feature is that there was a long time when you could redeem your notes, and not just the Silver Certificates, or Gold Certificates. Certain phrases such as "Redeemable in Lawful Money" were common, and to prove that these were Bank 'Notes', the phrase "Will Pay To The Bearer" told the person who had the note that they could indeed go and get 'Real' money for their paper notes. Few did so, as there was no reason to: people accepted the notes as money, and thus it became so. That's why the phrase "LEGAL TENDER FOR ALL DEBTS PUBLIC AND PRIVATE" is on our notes today. It is a declaration that it is money itself, not a check, a note, a certificate or a receipt of money.
All the notes in this photo below circulated at the same time!
1917 2 Dollar Legal Tender
1923 1 Dollar Legal Tender
1928 1 Dollar Legal Tender
1966 100 Dollar Legal Tender
1899 1 Dollar Silver Certificate
1923 1 Dollar Silver Certificate
1928 1 Dollar Silver Certificate
1922 10 Dollar Gold Certificate
1928 20 Dollar Gold Certificate
1929 20 Dollars National Bank Note

1918 FRBN
1929 20 Dollars FRBN
1914 FRN
1934 FRN
1950 FRN
1969 FRN