One of the guys still has the old colored flags on his pre-'91 gold Futaba, apparently too cheap to spring for a DuBro antenna marker. The numbers are correct.
He may have been using a Ham Band frequency on 53 Mhz and this is still the way to display some of those channels.
This is described near the bottom of page 19 of the 2005 AMA Membership manual, as well as the displaying of the channel numbers for 72Mhz.