Since they represent a transitional period, these 4 designs (and their numerous variations) should have a section all to themselves. They really don't belong in the first era since they were not created completely with Aircraft Factory, but they also do not really fit in the third era due to the fact that each one was originally thought up in 1996 like the first era, with the limits of AF5 in mind.  And although this group may seem small with only 4 designs, the airplanes that spur from those designs are quite numerous.  There are 4 versions of the ARX-5 Coyote, although only one currently released, and over 5 versions of the SR-75, most of which are planned for release.  So in total that makes 11 separate aircraft, 6 of which are in production right now.

Valkyrie II:
SR-75A Phase I - Single-seat Mach 3 testbed aircraft.
SR-75A Phase II - (SCA) Modified to carry the SV spacecraft for BOTA.
SR-75B - Two-place Mach 3 strategic recon.
SR-75X - Two place hypersonic missile test platform.

Aurora Project:
SR-75C - First stage of two, carrier aircraft.
XR-7 - Second stage of two, hypersonic recon.

Coyote:
ARX-5A - Lycoming 540 powered composite aerobatic plane.
ARX-5X - Orenda V-8 powered version.
ARX-5T - Pratt & Whitney PT-6A turboprop.
ARX-5V - Same as model A but with a V-tail.