Richard, I have one of those Chilton / OCee Ritch books- second print 1970- it's been very handy considering it covers so many models!
Phil, I also had a similar thought about just using plain shims as I figured that there is some 'torsional resistance' with the Thackeries in there,(Yay Bill you did get 'Thackery' right, the trees live another day
) and confirmed when I did it, rockers move so easily, but probably not enough to matter unless racing.
Also I agree with Bills suggestion about side-play, and could affect pushrod/ valve travel, but think 'solids would counteract this more effectively than Thackery springs, as they will just cushion it without removing it.
I reckon the Thackeries are just a quicker,easier and cheaper way of pumping out the product, as shimming requires a few seconds more calculating
We had chat about it a while ago, Muskrat will set you straight
Cheers
Ps Thanks Brian for sorting that out- Phil, there are still two on the outside under the nuts, but that's a different story