The 5 was added in the tire code box.
3L was 3.23 posi with metallic brakes aka 42:13 memo
70-9 is an offbeat axle designator but have seen a couple others coded that way in past few days of searches.
= Performance axle ratio with Posi (
3.23 .... ok )
64's are a messy bunch of codes from plant to plant
Like you said , too early for M21
Was this the "red ringer" 421cid magazine test car ?