It's well known that the approach you outline, i.e. a single-product 510(k) is informally assumed by its holder to extend to encompass a later-developed family of products that have similar techology and intended use to the original product, became pretty much the norm since the modern 510(k) era began in 1976. It also had become common for a 510(k) to be filed for a single product that was regarded by the filer as typical of a design family that was intended to be introduced simultaneously with the single formally approved product, with coverage for all products claimed under the same number.
Of course, even though this approach had been common, it's never been recognized in either the CFR or the FDA guidances.
We were told by a 510(k) examiner approximately 8-10 months ago that it had been decided at senior FDA levels that for products requiring a 510(k) that had been brought to the marketplace utilizing such an "informal family" approach prior to about two months before that time, enforcement action would not be taken, but that any products brought to the market subsequent to that point in time that were not formally included in a Traditional, Special or Abbreviated 510(k) before they were first marketed would be subject to enforcement action.
It is of course possible to get a Traditional 510(k) for a product family, or to extend an existing single-product Traditional 510(k) to encompass a family of products by means of the Special 510(k) process. Per the information told to us, that has to be done before marketing, though.