master cylinder

[Mr70]

That sounds like the original cylinder used by all 1970 Chevelle Super Sports,not just LS-6.Look for a "BB" prefix on the front machined pad too,and a casting date following the #409.
Also,look real closely for the assembly date code stamped around either one of the outlet bosses below each bleeder valve.It will usually look like 0123 for 1970,123rd day assembled.

[Ls6 Ragtop]

As Rick says look for a BB on the front machined pad for 70 Chevelle application. As for assembly date, most are stamped on the front pad along with the BB suffix for this application.
Very small font on the date, so can be pretty hard to see.
Chris