Dating items by patents - legit?

Some jewelry books have theorized that when you find the patent date of a jewelry design, that it is a good indicator of when the item was made and sold. Styles and designs changed often and so it makes sense that finding a patent would be a good indicator of when the piece was sold.

I've been wondering just how well that theory holds out and so have started some research comparing patent dates and advertisement dates. Here is one example, a Mazer pin, which really only brings up more questions than answers but give some clues, and has me hankering for more info!

In this case, there is about a year between when the patent was granted and when the pin shows up in the ad. What I don't know, is whether the pin had been sold all through 1941 and this is just the first ad that I happened to find, or was it not produced and sold until almost a year later?

This is a face pin from Mazer as shown in patent D123910. The patent was applied for in September of 1940 and granted in December 1940.

And here is the pin as shown in an ad from November 1941

I am sure there are those would say that dating a piece to the 1940-1941 time frame is pretty darn good! And I agree. But what if the companies patented an item years after it was produced? Or what if, once patented, it was sold across many years? (a more likely scenario in my opinion). I am intrigued to see what patterns I can find and will report more as discover more!