I recall this being an issue of the time, that the more recent, and better condition, Mk1 and early Mk2 stock, which had the 100mph Commonwealth or B4 bogies, was raided from the Midland lines in 1965-67 by the WCML of the LMR when the full electrification was introduced, while 10 year old dilapidated early Mk1 vehicles came the other way. That the last Mk1 rake in the country still in maroon livery was based at Derby into the mid-1970s (past comments here refer) tells a story of LMR priorities.
You could well be correct, but as my memory serves, the Midland did not have many - if any - of the early Mk 2 stock in those years in any case. My feeling is that the early Mk 2 stock (ie non air-con, and probably not even Mk 2C) only started coming in in dribs and drabs sort of 1973 time.
Again, not being dogmatic on this, my feeling is that by 1970, the vast majority of the Sheffield services had all 100 mph stock, mostly with Commonwealth bogies (even if they could not achieve more than 90 mph officially). The semi-fast trains were very much mixed some on B1s, some on Commonwealths, some, but fewer as I recall, on B4s.
And oh boy, did some of those on the B1s hunt on certain sections of cwr - I seem to remember one such section around Kegworth, north of Loughboro' - it actually felt quite dangerous at 85-90 mph.
I assume the maroon Mk 1s were all on B1s, but I never checked.
I'm almost certain the air-con Mk 2s arrived in October 73. I remember one early one morning that month, in LMR CM&EE building in Derby, crossing on a corridor bridge between the buildings and looking out of the window towards the south, seeing what must have been the 07.45 ex St Pancras to Sheffield arriving. It would have been about 09.00 <edit, no more like 09.30> hence the sun was low, illuminating the Cl 45, in pristine BR blue, and the air-con Mk 2s in tow - all gleaming in the strong light. And I thought, great! At last the Midland has air-con.
This would have been in the first or second week of their arrival. It was about the same time that Brian Clough left/was pushed out of Derby County, the city was in uproar, and I suspect industrial output at Brel Works and Rolls-Royce plunged 25% as a result. And I also suspect the Derby Evening Telegraph never noticed their nice new express trains to London as a result!