Exact same thing happened to me when trying to do my first cs problem set. It was some filename that I was sure had a 1 in it but it was an l. It's almost as if courier tries as hard as possible to make the two look the same.
Courier was intended to look like a traditional typewriter font. For many decades, most typewriters didn't have a key for 1 (one), you were supposed to use the lower-case l (ell) instead.
Teletypes did have a one key, probably because they were all-uppercase, so there wasn't a lowercase ell available.
Teletypes did have a one key, probably because they were all-uppercase, so there wasn't a lowercase ell available.