Calcium toxicity is a progressive disease. The birds most sensitive to it are hatchlings - they weight nothing, and are just putting their little bodies together. The damage done has a time component (how long were they fed excess) and a quantity component (how much excess were they fed). Its similar to smoking. No one can point at a given cigarette and say, with any confidence, "that one will give you cancer". But they can say, with the confidence of statistical certainty, that the younger you start, the more you smoke, and the longer you smoke, the more likely you are to get one (or more) smoking-related cancers.
Calcium builds up in the internal organs, and the damage it does is largely "invisible" until its quite severe. Excess calcium negatively affects feed conversion, leads to lesions on the internal organs, gout, urinary problems, and the like - but externally, until they are quite severe, you likely won't notice a problem. Thus the many people claiming its no concern whatsoever - because they've never seen their birds at full potential.
There are individual cases of a single bag of feed killing a flock of birds - but those cases involve manufacturing errors placing calcium at levels up to 10x intended.
Now that you are worried, the good news.
In broilers (similar feed consumption to a drake), at 12 weeks from hatching, clinical evidence of calcium toxicity was noted in most of the test flock in studies - but external evidence was near zero. The same experiment with birds over 26 weeks, clinical evidence was much less obvious (much less severe and much less widespread), external evidence was zero.
Assuming your drake was essentially mature, and that dosage was only a month, and that the layer being offered was likely in the 3.5-4.5% calcium range, I would expect that even if you were to butcher him today and poke about his organs, you would be hard pressed to find any signs of physical damage, and no measurable long term effects. The short term effects of reduced feed efficiency due to impaired nutrient absorption you've already paid for.
In the minds of some, the above is "head spinning around and popping off".
