The YRF-84K was redesignated from the YF-96A when modified and used for initial FIgher CONveyer (FICON) testing with GRB-36D. The YF-96A aka "first" YF-84F was modified by addition of a fixed hook mounted on the nose which enabled the aircraft to engage the capture trapeze deployed from the bomb bay of the GRB-36D mothership. Also, the nose probe was removed from the nose intake splitter, the V-shaped windscreen was replaced by the production flat-style windscreen, and bracing tapes were added to the canopy. Subsequent to testing with the YRF-84K, FICON became operational for a short time with 25 RF-84K aircraft based at Larson AFB, and the GRB-36D aircraft operating out of Fairchild AFB, both bases adjacent to each other at Spokane, Washington, with RF-84K aircraft. It is noteworthy that this aircraft, although having an "R" designation for recon, was not truly a part of the recon Thunderflash series, as it had neither the recon nose nor the lateral intakes. It seems likely the reason for the "R" in the designation was because it served as the test vehicle for the RF-84K. It currently resides in FICON form at the USAF Museum, Dayton, Ohio.
An aside to this discussion: One of my brothers was stationed at Fairchild AFB during the early 1950s. During one summer (either 1952 or 1953), my family (including me) visited him there just a few days after a B-36 had crashed while landing and burned. He took us to the flightline, where we were able to see the burned out hulk only a few hundred feet away. The largest remaining parts of that giant bomber were the six lumps where the remains of the engines lay, along with the mostly intact vertical tail. Quite a sight for this young boys eyes. However, he left the Air Force late in 1953, approximately one year before the FICON program was started, so was not at Fairchild when the program was operational.
YP-96A with FICON hook on nose and 26 degree anhedral on horizontal stabilizers.
YRF-84F 49-2430, formerly the YF-96A aka "first" YF-84F, attached to the trapeze under the GRB-36D during FICON tests. This photo was apparently a widely distributed publicity shot, as it has appeared in many publications. I obtained this photo about 1956.
Links to photos at other sites
USAF Museum YF-84F -- Page
USAF Museum YF-84F -- 3/4 Right Front
USAF Museum YF-84F -- FICON with B-36