There's a slight variation on the PMG rule (at least, they won't annotate repeater unless it's at least 6 digits total). Here's what they say: Repeater Serial Number--This serial number will repeat. The minimum number of digits that need to repeat is two (IE: 121212). Likewise, the minimum times the sequence needs to repeat is two (IE: 12341234). <--based on 'repeat is two' 1212 ought to be a repeater, right? Nope.
Two examples, the Baltimore $5 gets the note "Repeater Serial Number", however, the Portland note doesn't (it ought to be a binary repeater in my opinion). 4 digits appar