I've had it as a doorstop for a few years.

[Robby Raccoon]

"Armstrong's DP 1" is embossed on the front heel, "MADE IN U.S.A." then a small circle with an uppercase A, then, "7 50" It's a clear glass insulator I purchased for a dollar back when my interests were more piqued in these. Once more my interests are blooming, and I know it's not special, but how often does one come across these and what year is it from? I moved it from the basement where it was holding the door from slamming. I know, you're thinking, "Hmmmm. Glass insulator to stop a door slamming it into an unforgiving concrete wall. Oh, great idea!" Well, they were built to last and I've never had an issue, no matter how hard my family members slam it. Forgive the low-quality pictures. I'm fairly sure it's a bit heavier than a standard Hemingray-42. The bottom was smooth. No teeth, no drips.

 

[Robby Raccoon]

I also used, as a paper-weight, a "WHITALL TATUM CO. No 1 / 27" with the O in NO small, put higher up, and underlined. Opposite side says "MADE IN U.S.A." and has an inverted triangle with a larger W over a smaller T for the company's name. Smooth bottom, looks identical from a distance to the other one aforementioned. Clear. Know an age? I did research. Once. It's been so long and I lost interest in these things sadly. I have one more that says '33' instead of '27' used as decoration. What's up with that?

 

[sandchip]

You might enjoy this site, Robert. http://www.insulators.info/

 

[BillinMo]

This is the successor to the Hemingray 42 -- it's basically the same shape, with the fine edges beefed up to make it more rugged, introduced around 1938. The Armstrong example here is a little later. The 7 would be the mold number, and 50 the year the mold was made. It's tough to say from the photos posted, but I would guess this was made in 1950. If there are dots, add a year for each dot. Armstrong cranked these out in mass quantities and both Bell Telephone and Western Union used this shape as their standard insulator up through the 1970s, so they're not going to have much collector value. The Whitall Tatum example Robert mentions is an earlier one, but Whitall Tatum didn't always use date codes. The 27 and 33 are mold numbers, not dates. It would probably be from the late 1930s.

 

[Robby Raccoon]

Thank you, SandChip, for the site that I will certainly check out!
And thank you, BillinMo, for the information you provided!
I had one more intact insulator, but it's missing. But! I found my other W.T. 33! None of them have dots outside of punctuation use.

 

[Robby Raccoon]

More photos:

 

[Robby Raccoon]

One final photo of three of what should be five insulators in my collection:
I hope to get a couple more this year.