I believe the ver. of 'Modern Times Rock N Roll' that JPJJD was referring to was from the FIRST time Queen played at the Rainbow Theater, March 31st. 1974. The Rainbow show that is on video (released on Box Of Tricks) is from their shows on Nov. 19/20, 1974.
However, there IS video of 'Modern Times' (and 'Son and Daughter') from the first Rainbow gig.