I haven’t checked this in a while, but I was at the shop this morning setting up some Crane ductlles on a customers engine, and reminded me to go check the stash. As noted, the 273 are always going to be straight. The usual dead give away is the boss area where the screw goes. It’s rounded, with no “ angle on the tail”. The Isky is very easy to spot, as it has a huge pad, larger fulcrum arm, much thicker “ nose” over the top. Also, the underside of the body is very smooth compared to the OEM styles, which have a large nub on the bottom, or the appearance it has been machined off in the case of the RB style.The Isky is much beefier from the casting up the the arm as well, significantly more beef. The downside, more weight over the valve tip. I have ran a set of those on a old bracket racers 440 with the old .750 Lunati S/S cam for over 20 years with zero issue. Never bushed, never lightened and fairy heavy springs. Good pieces IMO. Someday if life ever slows down, I’d love to make a jig to bush them and index etc.

Pic 2