[DeTomaso] M-6564-C351 pedestal mount rocker arms question

[Doug Braun]

I was looking at the 2006 FRPP catalog (page 100) at their M-6564-C351
pedestal mount rocker arms for the 351C.  Under "Applications" it says

"351C/351M/400
1973 and later
429/469 production
cylinder heads".

Does this mean they won't fit 70-72 351C heads?  Why would that be?  Has
anyone used these on pre 1973 heads?

Doug Braun
blue 73L #5505