I am a brand new member. This category had nothing in it so I thought I would add something. I have a pair of Signal Corps P-11 phones. I think these are not uncommon. They are very similar to the Western Electric Type 509-W but earlier. I got curious as to how old they might be and was able to approximately date them from the patent notice. They are marked "Patent applied for". Later WE phones of the 509-W type carry the patent date of July 23, 1918 for patent number 1,273, 351. The actual patent number is shown on later phones. Now, this patent was applied for July 20, 1916 and the patent was issued on July 23, 1918 so the phones must have been made in that interval. They are about ninety-five years old and still work fine. The sensitivity is as good as much later WE phones of similar type. The original cord was a heavy rubber-covered one that disintegrated years ago but, by pure luck, I found a replacement. I think I bought these phones at a Los Angeles surplus house called J.J.Glass probably more than fifty years ago (giving away my age here) and have used them ever since. They are perfect for CW.
FWIW, my understanding, which I have not been able to confirm, is that the rules about patents changed in 1927 to require the actual patent number to be recorded on the device rather than just the date of issue. Patents are issued in lots periodically so its sometimes difficult to determine which patent is referred to purely by date. The existence of a patent date or number is only a very rough guide to date of manufacture of something. Until recently U.S. patents were good for seventeen years from date of issue, I think it is now twenty years. I am not sure of what rules applied to the indication of a patent on something after the patent had expired. If something has a patent _date_ only on it its likely manufactured prior to 1927 (assuming my information is correct) but the existence of a patent number does not indicate it was made after that time since one could use either date or number prior to that.
Any US patent ever issued can be found on either the US patent and trade mark office site or from Google Patents. The latter allows full text searches for all dates where the USPTO site is limited in text searching to patents issued after 1975.

RK