[blucar]

From my limited experience with Y block engines, which have been with 292/312 '57/64, all of which have had the 7/16 adjusting nut like feature...
The adjuster is very tight and hard to turn, using a 3/8 drive socket with the engine running, slowly turn the socket to adjust. The feeler gauge is a special "steeped, go/no go".. I use to get mine from SNAPON,, Still have one in my tool box.
The early style with the lock nut and screw driver slot are more difficult to adjust on a running engine.
I think in the long run the nut type are less trouble..
A little info from experience on the Y Blocks... They have terrible oil passages through the heads, i found this out the hard way in the late '50's, I always used high detergent oil, 20/50 Castrol GTX.. I historically got over 100,000 miles out of an engine, had one that went over 500k on two rebuilds..