"Isn't that why a bird meter is the best way of seeing true power?"
no, the H slug (2-30Mhz.) and the A slug (25-60Mhz.) will both respond to nominal power levels outside of their respective ranges. (not to mention the fact that the second harmonic falls well within the range of the A slug) it has to do with the manufacturers stated % of accuracy, not some imagined immunity to harmonics. you'd know that if you actually knew how to test for it and you don't see Bird making the claim because it isn't true. it's a detector circuit, not a filter.
"the cobra 29 i had WOULD DO 47W ON FREQ.!! with very little bleed to adjacent channels, if any at all. i tested it thoroughly."
then you should be able to fire right back and tell us all what the total width of both sidebands were when the transmitter was modulated since you tested it thoroughly and this parameter combined with modulation index establishes the amount of energy that would or would not contribute to adjacent channel interference when the transmitter is in operation. since the mod index is already approaching a factor of 6 (5.875 for 2W Carrier / 47WPEP) your statement already smacks of BS and total sideband width is irrelevant.