I also have a Mitchell cover that is made of a softer cotton twill, not the smoother poplin material
are cotton twill covers a sign of early manufacture?
I don't know about that, but the stamped info will give you date info.
I just checked my own Mitchell cover it has a a 1969 DSA contract number whereas the 1959-61 (approximate) models have a simple four digit contract numbers such as "CONTRACT NO. 8027" and the FSN 8415-261-6833" numbers. The DSA contract number sequence changed a bit over the years, but it always starts with "DSA" and often included the year in plain sight: mine has the contract number "DSA 100 69 0 1921" which is a 1969 contract. It also still has the FSN number "8415-261-6833" so apparently that was not replaced by the DSA number, instead it was the four digit contract number that was replaced.
One thing I don't know is what kind of overlap there was between the DSA contract numbers and the old style - did everything produced in 1962 have a DSA prefix or was there still some use of the old "CONTRACT NO." prefix?
By the way, my 1969 Mitchell cover appears to be poplin.