Aftermarket cast iron rocker arms are mentioned in a lot of the early-'60s Slant-6 buildup articles. About a decade ago I found a set, new old stock in a mouldering box. Snagged 'em, cleaned 'em up, and stashed 'em.
The box says "1960-62 Valiant Lancer Plymouth Dodge 170 225 6cyl 1.6 ratio Intakes Only". I'm not sure why the admonition not to use them on the exhausts. Perhaps the thinking was that rockers like these would only be used with a camshaft much biggger and compression much higher than stock, and using these 1.6:1 rockers might cause the exhaust valves to smack the pistons...? Seems odd to me. The old buildup articles show them being used on intakes and exhausts.
The casting quality is really nice and I might've installed them on one of my engines the last time I had the top end open, but I noticed that these rockers have only one oil passage, from the shaft bearing rearward to the adjustor/pushrod area. There's no oil feed to the front of the rocker. That's no good for valve tip/rocker tip lubrication. I also thought to add the missing passage, but that seems like very delicate work, and I'm not familiar enough with casting intricacies to assess the risk of breaking or otherwise ruining a rocker in the attempt to drill the forward passage.