Ah, yes -- the "Bird". Probably talking about the Model 43, which is a decent, rugged meter which is still only accurate within ±5% of FSV. And if you add the "Peak" kit, making it a Model 43P, it's only ±8% FSV. And like any other measurement device, it requires periodic calibration - along with any slugs used with it.
The Bird 43 is not a "lab standard"; however, many seem to think it's the touchstone for "accuracy".
There are no user adjustments on the slug or pickup of the Bird 43 that I am
aware of. The 43 may not be a lab standard for some but it is certainly more accurate than any wattmeter in any amplifier that I've ever seen or owned. The only manufacturer that I am aware of who used a Bird wattmeter in its amplifier was the old Henry 3K Ultra.