Various people have reported similar markings on multipart porcelain insulators that we still have not been able to confirm with 100% certainty. The marking is always weak in one area or another making it impossible to determine the date. However, with several reports I believe that I can now make a good guess. 

The most complete marking was reported by Mike Jordan (shown above). It was on a three-part insulator with "U.S.A. PAT. " underneath and a date under that. To avoid confusion, I have shown what I believe to be the correct date. Mike's marking was the clearest, however, he said that he was not sure of the date. It looked like "2-20- 20". This date is not a Tuesday. Since patents are only granted on Tuesdays, the date is obviously wrong. The insulator is M-2611 with a reddish-brown glaze characteristic of Locke manufacture (photo below).

Mike Guthrie reported a similar marking on M-2614 with what he guessed was the date "6-23-10". That, too, is not a Tuesday. Robin Harrison reported "A.PAT" markings on M-2614C and M-3541 each with the Locke R=oo insulator logo.

The M-2614C had marking "A. PAT. /1-2?" under the insulator logo and the M-3541 had the marking "A. PAT. /??-20". Chris Hedges reported the marking "PAT / 4-22-2?". So, you can see that we have little to go on with exception of Mike Jordan's nearly complete marking, and at first glance you might think that none of these markings are related.

We can assume that the patent in the marking was a patent assigned to Locke patent since the insulator logo marking was Locke's and the specimens appeared to be of Locke manufacture. Next check the similarities of the reported dates keeping in mind that with weak markings, reported numbers could easily be in error from the actual date:

Even with all of the poor incuse date markings, it is obvious that the first digit of the day is a "2" and that year is probably in the 1920's. I checked all of the Locke patents for the 1920's with a Tuesday on the "2?". Reissue patent #14,888 seems to fit with a patent date of 6-22-20. A copy of the patent drawing is shown at the end of this article or you can turn to page 113 in my book Multipart Porcelain Insulators . This is a reissue of patent #1,311,067 granted to Walter T. Goddard for a multipart insulator design which was similar to the Faradoid patent #1,374,998 assigned to Westinghouse (see my book on page 114). [Note: if there is a minor error in the published patent, a reissue patent number may be assigned by the patent office to correct the error.] 

If you study the reported patent dates, you can see that they are indeed variations of the true patent date given the legibility of the weak markings! So, I am 90% sure that we have solved this mysterious marking. As a hint, if you have a porcelain insulator with an incuse marking that is difficult to read, take the insulator outside in bright sunlight. This will make the weak marking more easily read.

Wayne Elworth sent me several photographs of an advertising promotional paperweight that I have not seen before now. It is made of porcelain in the form of a three-part insulator similar to M-3440 and with "LOCKE" in large letters embossed on the top skirt. It is about 3" tall with chocolate brown glaze (see the two photos above). I would guess that it was made circa 1927-1930's.

There are many variations of insulator company giveaways. These are typically called "salesmen's samples". Most would serve a useful function such as ash trays or coffee mugs so they would always be visible on someone's desk. Locke made many thousands of coffee mugs over the years, but they were permanently discontinued last year. I have a bronze bulldog standing over a multipart insulator sitting on my desk at home. It was made for the Illinois Electric Porcelain Co. The tradename for their line of nail knobs was "BULLDOG".

I recently was going through, a 1928 Lapp catalog. I found references to three patents that I did not have in my patent file of more than 1045 insulator related patents. By the way, I have copies of all 1045+ insulator patents. I will send you a copy of any of these patents for $1.00 postpaid and unfolded. 

I also found some interesting information about Lapp glazes and markings. First, the Lapp underglaze ink marking which includes the date (see example above) is not used on every insulator. Small insulators are thus marked about every fifth piece; larger pin type insulators and all suspension types are each marked. The purpose for marking was, "You thus have the year of production, which in nearly every case will also be the year of erection imperishable recorded in your stock, and on your line to give a basis for service record."

There were a variety of glazes offered:

lam not familiar with availability of Lapp insulators in various glaze colors. Several unipart types with pretty light blue glaze have been reported. This may be their "light blue" rather than the "blue". I am not sure if I have ever seen a white Lapp. Even chocolate seems to be uncommon compared to the uniform pumpkin Lapps. It is not surprising that Lapp persisted for many years with the pumpkin glaze as it was pretty and easily displayed their famous underglaze ink markings.