I may have found a little more evidence regarding these symbols, so thought I would share again.
I've given the original German text also, as my translation is only ever courtesy of Google
Which seems to translate as:
"Pre-calculation of Altitude Performance
By Otto Schwager
For the manufacturer of aircraft, it is often of great importance to know the altitude performance of an aircraft engine, since the climb times and flight speeds to be guaranteed at the various altitudes depend on this. The most reliable way of obtaining the performance at altitude is to examine the engine in the vacuum chamber under the actual pressure and temperature conditions encountered during altitude flight. Unfortunately, there are currently only very few such vacuum chambers, so that it is not possible to record the performance, throttle and consumption curves for every type of aircraft engine in the same way as the altitude performance curve. As a result, people make do with the pre-calculation of the height performance and use a wide variety of formulas for this, some of which are theoretically based and some are based purely empirically on test results.
It can be assumed with sufficient accuracy that, given the same mixture ratio and the same speed, the indicated engine power is directly related to the air weight, and also that part of the power loss at sea level remains the same for all altitudes, while a second part changes in relation to the air weights.
It denotes:
No the braking power at,
yo unit air weight,
Nz the braking power in,
Z km altitude,
yz the air weight at height Z,
to, To, tz, Tz the air temperatures at 0 and Z km altitude,
bo, Po, bz, Pz the air pressures at 0 or Z km altitude,
nm the mechanical efficiency,
a the variable part of the power loss,
b the constant part of the power loss,
NRo the power loss at yo,
NRz the power loss at yz,
Nio is the indicated power at yo,
Niz the indicated power at yz."
(excuse the subscripts being too large)....... So, seems to agree with the sea-level power correction.
I've also attached the entire document, there is a series of these factory aircraft engine newsletters available at address below - some very interesting articles from BMW pre-WWII.
Files Archive - E28 Goodies
Thanks to all for their input; always appreciated