Fumblin, that 61 threw me too. Must be a typo. My parts manuals list all of the engine serial number suffixes from '56 thru '65.
This site lists inline 6 suffixes up to '82. The general format is Plant/month/day/suffix (w/o the slashes) for '57-up. For '57-'59, the month could be 1 or 2 digits. For '60-up, all months were 2 digits.
