NYC Duplex Postmarks

Do the letters "H" and "K" in these 2 duplex postmarks refer to NYC stations or simply postmark identification.

I see that both cards with same address also have an adjacent CDS with a prominent "D" which I tend to think represent a station.

Yes, they are station indicators. NYC has many station and sub station specific postmarks. Sub stations are usually show numbers rather than letters.

These threads may also be helpful:
https://goscf.com/t/71937
https://goscf.com/t/51505&whichpage=1&

Thank you John and Russ for the information

Can someone tell me the location of NYC Station X at the time of this postmark. In addition period of use would be welcome. Thank you.

About half way down the first link I posted on 10/10/2022, there are several scans from the Postal Guide volumes including 1907, which has a different location for Station X than listed in the July 1, 1917 Postal Guide, which shows Station X at 373-5 E. 138th St. I do not know when they moved or if there were intermediary locations between these two. The Postal Bulletins would have this information.

As an aside, "X" cancels like yours might make an interesting thread - and not the manuscript ones!

Just to add, that address is in the Bronx.

The Postal Guides and supplements have been digitized and are available through the Stamp Smarter site.