I believe the NCAA had a rule in the 60's and 70's that all uniform numbers had to be 2 digits, and no numeral higher than a 5 could be used for either digit. It was to allow the officials to signal to the scorer's table which player committed a foul, and since they only had 5 fingers on each hand, the could not handle numerals higher than a 5. But eventually, they removed that rule.
Also, I recall seeing pictures from my programs and yearbooks from the 50's and earlier that teams Bradley played against often had players with single digit numbers, including 6, 7, 8, and 9. However, I am not in town, and don't have access to those old yearbooks, so I can't check specific numbers until tomorrow.
(But I do recall looking this subject up last year, and I will have to check this, but I believe I remember finding that Tyrone Cole-Scott may have been the first BU player to wear a uniform with just the single number 0, though someone else may have worn 00 before.
Several players have worn the number 1--Michael Stewart wore it, then Montana Heirman it in 2004-2005, and then Saihou Jassey wore it in 2005-2006 and 2006-2007.)