Yes. It’s a left hand letter.
This would be my guess as well. It was a mistake (either in the original code, or in typing up the web page).
By the way, if anyone is interested, it’s a pretty clever trick. Basically, the perl script took the list of words, and applied a rule that puts each one on a line twice - once in lower case, and once in upper case. So
abc
adg
becomes
abc ABC
adg ADG
The tr section replaces the capital letters with the corresponding number. We then have
abc 111
adg 123
The grep looks for all words that have six letters the same in a row. The dot matches any character. The \1 refers to the string that was in the parens. The parens themselves are ignored.