Here's my SWAG based on production numbers and assuming even distribution across solid lifter engines suggests around 8.5% of the Z's could have had an M22.
Here's the logic - In 1969 M22's could only be ordered with solid lifter engines.
So, '69 Production Count of Solid Lifter Engines:
COPO 9560: 69
COPO 9561: ~ 800
Assume 30% of the L78/89's & COPO's came with automatics or about 1820; balance of 4,248 had manuals. NOTE: THIS IS A SWAG....I do not have data to quantify this guess. If its less, just fewer Z's had M22.
Net total number of solid lifter engines that had manuals that qualified for M22: 24,503. Again assumes 30% of the big block solid lifter cars had automatics.
Total M22 production: 2117
% of total solid lifter cars that had M22's: (2,117 M22's)/(24,550 solid lifter engines) = 8.6%
Again, assuming M22's were evenly dispersed across solid lifter engine Camaros in 1969, would indicate a little over 8.5% of the Z's had an M22.
Lots of assumptions here and nothing absolute, but might be indicative....of course, if the bulk of the M22's went behind L78/L89's and COPO's, then the Z % with M22's could be quite a bit less.
By the way, my Z came with the M22. Dealer car sold off of the lot per the original owner when I asked him. No idea why the dealer opted for the M22.