Tracking American Open Skies OC-135B/W Aircraft

"An OC-135B aircraft sits on an airfield at Ulan-Ude, Russia prior to an Open Skies flight.
DTRA conducts inspection flights with the U.S. Air Force in accordance with the Open Skies treaty."
(DTRA photo - March 3, 2009) Flickr

The United States operates two Open Skies Treaty-approved planes designed to perform observation missions over Russia, or other countries.  The aircraft are based out of Offutt AFB for ops, training and maintenance, and function as part of the 55th Wing, 45th Reconnaissance Squadron (per Wikipedia).

The United States Air Force does not publish press releases, or mention to the media, when they perform Open Skies Treaty overflights; to keep track of the flights using OSINT, adsbexchange.com and flightradar24.com can be leveraged.  

Using Google, the tail numbers became clear  (Registration / ICAO):

Boeing OC-135B Open Skies (61-2670 / AE08D5)
Boeing OC-135W Open Skies (61-2672 / AE08D6)

While the transponder beaconing from 61-2670 broadcasts that it is a Boeing OC-135B, 61-2672 reports itself as an OC-135W.  Contradictory information online suggests both planes are of the OC-135W variety, but I can't find the differences between one and the other.  It could be a cosmetic difference, I'm unclear what the difference in designation would be between the OC-135B and OC-135W.  Regardless, if you sign up for an account on FlightRadar24.com you can program an alert to be emailed to you when the flights are detected by one of their global network of receivers.

If you want to look at historical flight information, adsbechange.com may be more useful for that task (and free!).

Here is a direct link to a dynamic report containing the most recent OC-135B/W flights (it takes a while to load, hang tight)

Callsigns

There is some confusion about the callsigns that are used, and it comes from the historical report mentioned above.  It seems that when an Open Skies overflight mission is underway, the callsign used, for either airframe, is OSY12F (does F stand for overFlight?), and when the plane is in transit the callsign is OSY12T (does this indicate "in Transit"?).  

The two planes also use other callsigns; and I'm not sure what they are otherwise commonly used for, if they are indicative of any specific operations that are not Open Skies related.  

From October 13th 2015 and September 17th 2016 I noticed the following callsigns being used by 61-2670 and 61-2672, as documented on adsbexchange.com

COBRA12
COBRA23
COBRA24
COBRA32
COBRA35
COBRA43
COBRA44
COBRA45
COBRA46
COBRA52
COBRA53
COBRA55
COBRA70
COBRA72
SABRE35
TORA82
TRUBS72

...suggestions as to what operations these callsigns might be indicative of, or confirmation they mean nothing at all, would be welcome!  ( Some callsigns have been noted previously from 55 Wing / 45 Recon here and here, but do not indicate if they suggest a specific function (training/deployed ops/etc) )