According to this post on this topic, the SA76RD (along with the very similar SA67D) succeeded the SA76RC (which in itself was a successor to the SA76RS (same as the RC, but with an electromechanical horn), though I’m honestly not sure what the exact differences are between the SA76RD & the SA67D (maybe if a veteran smoke detector collector shows up here they can give us an answer or two to this).
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