I have heard this carb was only used in 1915. How true/accurate is that?

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Looks like a late Holly G carburetor. The choke has arms for two choke pull wires. One out front for crank starting. The other to choke the engine from the seat, for use of a starter. So about 1919. I have several Holly Gs, most of them have only one arm for a choke pull wire.
Drive safe, W2

Erich:

Your choke butterfly is 1919. If the rest of the carb is all brass then you have a 15 and someone put a different butterfly in it. If the body is brass and the bowl is steel you have a late 17 or 18.

The carburetor below came off a very original 1919 Center Door.

Erick:

I forgot to say that the top embossing says the same thing Dec 22 1924.

Erick:

I meant to say Dec 22 1914.

Erick:

I looked at your lower picture again. It has the DEEP WIDE RING around the choke. This should make your carb a 1916 and early 17. with a 1919 butterfly.

Any real difference between these and the NH? Or about the same?

I have never used one. It probably has a cork float which can be a bit of a problem to restore. All of mine warped or shrunk enough with age to be problems. There are ways to fix or replace them, but I am not the one to answer that one.
Drive safe, W2

I believe your Holley "G" is unique to 1915 with that collar around the choke. It has a later pressed steel choke arm for inside choking required with electric starter beginning in 1919. Performance wise, I have changed out my Holley "NH" carbs for the "G" for 2 reasons: easily cleaned passages using a paper clip and performance which gives easier crank starts and smooth idle. Just be sure to replace the old cork float with a modern one for about $8.00.

Erich, I saw one of those being used a couple weeks ago, so they definitely were not used just in 1915!

With that said, I never liked the Holley G carbs for two main reasons:
-They have that pot metal tube that is usally blown apart, and generally is not available.
-They have a cork float.
If there was a good aluminum replacement for the pot metal tube (that fit right and the vendors always had in stock), and a brass float was available, I would change my mind, because if they are in good working order, they do work well. I just see them as a pain to rebuild. By the way if anybody has any good rebuild-able NH's they want to trade for G's, let me know. I would prefer a face-to-face trade to insure conditions are equal.