Ooops- Ignore my last post above.
I had got the bits of the curve the wrong way around.
The correct way around is-- the portion to the LEFT of the "knee" is where the throttle is fully open. The "steep" bit on the curve is the constant power portion.
Rechecking a few figures shows that where the performance of the machine is unlimited by engine power (to the RIGHT of the knee in the R22 HP and Alpha OGE curve) the weight is constant at constant DA
For example 1000 feet at +30 is equivalent to 3000 feet (ish) at ISA and 5000 feet at -10. All have a max weight of 1340 lbs.
God-- How embarrassing!!
Off to eat Turkey (american visitors staying) and humble pie