I found a Mesa catalog from 1984. This is the catalog that introduces the MK III. The speaker choices were as follows
Mesa Black Shadow MS 12, this is the 150 watter made by Eminence. There are 2 versions of this speaker. One with a smooth cone, and one with a ribbed cone. Smooth cone is the 1st version.
Vintage Black Shadow 50 watter made by Eminence. Stock speaker in the Studio 22.
Celestion Black Shadow, 90 watter still in use today.
EVM Black Shadow, 200 watts This is an EVM 12L with Mesa's label on it.
Unfortunately I don't have a price list from this period. But if I remember correctly from my Boogie retail sales days the MS 12 was considered the stock speaker. I don't believe there was an up charge for the Celestion, but there was an upcharge for the EV. I think it was $100 extra for an EV.
So the MC 90 has been in use since at least 1984. The EV was available until last year when EV stopped making it.