Jan F. Homma @ F27 Friendship Association said:
F-27C
Right from the start Fairchild did try to get their hands on defense orders. But especially in 1960 and 1961 did Fairchild mainly concentrate on military orders for the US Army, US Air Force and US Marines. The four military versions of the Fairchild F-27C (an F-27A with cargo door, which, strangely enough, never was certified) were summarised under the Model M-258.
The M-258G was in fact an F-27A with cargo door and powered by RR Dart 8 engines (military version of the RR Dart 7 Mk-258) and with military communication and navigation equipment.
The M258H was the same as the M-258G, but equiped with double slotted high lift flaps and a fully movable horizontal stabilizer (STOL). Fairchild had plans to buy Fokker's F1 prototype.
The M-258J was the same as the M-258H, but had a cargo door at the rear of the shortened fuselage (rear loading)(Fokker F27MS). And the last M-258K again was the same as the M-258J, but powered by 2 General Electric T-64-GE-8s. Also, a version with Lycoming T-55 engines was developed. But it was all in vain. Not one order resulted.
[...]
They still had expectations for a defense order viz. an ASW version for the US Marines model M-294A, a STOL version for the US Navy and a standard version for the Strategic Air Command. A single order from these authorities would mean an assignment for minimal 100 units for Fairchild.
[...]
In the beginning of 1961 PanAm wanted to buy 5 Fairchild F-27As, so they could start a new airline, somewhere in Africa; this by order of the American Government. But the mandate did not materialise, even if an option had been taken.
Not a single United States Defense order was secured by Fairchild, in spite of a lot of effort and a lot of time. The lack of such an order became in fact the deatblow for the F-27 production in the US.