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Check also that they are the same ratio, I had a high ratio (1.54) and low ratio (1.43) rocker on my 57 when I purchased it and it always ran funky and could never get rid of the pinking with timing adjustment, I fitted another high ratio rocker and it ran a lot better and timed up without pinking.
Anyway the high ratio had the interference nuts 1/2 head (i think) and the low ratio were 7/16 head adjusters. I have also seen the slotted screw type with jamnuts but only in low ratio rockers.