Chuck, and CRs:
I suspect that the newer Campy SR cage plates were redesigned (optimized) to help shift better on larger freewheel cogs, and with later chainring setups (42-52 compared to 45-52).
My reasons (analysis):
I don't know all of the original Record steel RD design nuances (without checking your Campy timeline and The Dancing Chain) but I recall that the original Record rear derailleur had been designed in the era of 151 bolt circle cranks (45?-52) and long rear dropouts.
The older, longer rear dropouts allow the rear wheel to be set further back to help the RD shift onto larger freewheel cogs, and the smaller chainring difference (compared to the newer 42-52) results in a lesser change in top pulley height (and more even top pulley to freewheel clearance).
In 2001, when I was setting up my new Mercian with a 1984 Nuovo Record rear derailleur (and shorter modern dropouts), I couldn't shift into the 23 cog when on the 42 chainring, while using a 54" chain. The top pulley wheel was rotated too close to the bottom of the 23 cog. I could shift into the 23 when on the 52, because the pulleys were angled away more and thus cleared the bottom edge of the 23. If I shortened the chain to 53", then the 23 shift (42 c-ring) worked OK, but the chain tension was more than I like when on my favorite ratios, 52x19 and 52x17. (Chainstays are 41.3 cm)
I prefer to use a 54" chain because the NR RD pulleys are approximately vertical when in the 52 x 14 gearing. I was taught many years ago that having "vertical pulleys" in first gear and top gear was the 'correct' NR RD setup.
I didn't have any problems with the same chain length on my Pinarello (using a Super Record RD and the same gearing) so I did a side-by-side comparison of NR vs.SR cage plates. What I found was that the spacing between the pulleys is identical between the two designs, but the later SR plates have a mounting hole (pivot point) which is 1 cm higher that the NR plates.
This pivot change lowers the pulleys by 1cm, which increases the distance between the top pulley and the freewheel cogs. The increased pulley to FW clearance allows for an easy shift into the 23 (42 c-ring).
Of course, having the pulleys further away will degrade shifting on the smaller cogs. (In comparison, the 1970s Japanese slant parallelogram design maintained close even spacing to the freewheel cogs.)
I suspect that the extended top lip on the outer SR cage plate was a design change to help compensate for the increased clearance between the top pulley and the FW cogs. Ted Ernst suggested to me that the extended top lip would help shifts into larger cogs.
With the SR cage plates (on a NR RD body) I really have to fuss to get a decent shift from the first position cog (13 or 14) to the second position cog (15). I often miss it and hit the third position instead.
The NR plates provide a better shift on the smaller cogs, and they're thicker, so they don't flex nearly as much, either.
The winter weather has prompted me to read the Paris-Roubaix anniversary book while eating my dinner. I guess I'll have to start reading The Dancing Chain, too!
Andrew Gillis (Christmas lights are up in Long Beach)