It's running UNIX Sixth Edition, a.k.a. V6. Yes, that's the one John Lions commented in his famous book. boot 
STTY -LCASE
This are the manuals from Sixth Edition Unix.
Thanks to aiju for helping getting this set up.
site
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Unix was the first system I used that expected to interact in mixed case. The Model 33 Teletype and its emulators were popular timesharing terminals in those days and they only wrote in upper case.
Unix would simulate lower case by converting upper to lower as part of input canonicalization. One had to slash-escape terminal input where upper case was actually wanted. What a pain. Hence the custom of all lower case everywhere.