[Mr70]

They're Repops.
George thats a McCreary tire code which reproduces that tire today and has been as far back as the early 1990's.
That's showing a 33rd week of 1991 manufacturing date.
McCreary tires are marked with an eleven-digit DOT code. The first four characters identify the McCreary plant number,the next four characters identify the McCreary mold Number (Part No.)..the next two digits identify the week of the year and the last digit indicates the year of manufacture.
For example CYLD F4K5331:
*(CYLD) is the plant number.
*(F4K5) is the mold or part number.
*(33) is the 33rd week of the year.
*(1) for 1991.

1967-70 Chevelle Super Sports used these tires originally.
More common on the 67-69 models,and were extremely scarce on 1970 Chevelles,but a hand full did receive them,(I believe 8),on the assembly line.