Later version of the Pioneer RT-701.
Unlike its original, the RT-707 isn't rare, at all.
Pioneer sold thousands and thousands and thousands of them worldwide : when the reel-to-reel market was beginning to stop its continuous rise, a small-sized recorder was a better option than a full-tilt RT-2044 or Sony TC-880-2.
Size-wise the RT-707 was far more successful at being a small reel-to-reel than ELCASETs were at trying to bridge gaps.
The RT-707 is an RT-701 with auto-reverse and with independent L/R record switches - therefore with a repeat switch next to the pitch control know, with a direction switch next to the pause button and two play head azimuth adjustments on the head block.
Also added from the 701 are output level pots, DIN rec/play and switched AC terminals at the back. Putting the output levels pots on the front would've made a bit more sense but the RT-707 was an upped 701, not a new product - got to make some penny savings somewhere.
However famed, the RT-707 started as a fairly simple recorder with a minimum of features, packed in a very attractive design - the RT-701. That is probably the reason why Pioneer remained fairly discrete about it in Japan : if pretty as can be, the RT-707 nevertheless is a basic entry-level tape recorder.
It is sales manager throughout the world which, after discovering the RT-701, probably asked for something more meaty - enter the RT-707 and, later, this time quite above the entry-level of the 701/707, the much bigger and most magnificent RT-909.
Both models remained available much later outside Japan, being advertsied as exciting products as late as 1981 in the USA or Germany when they were relegated to the back pages of the catalogs since 1979.
I can see the RT-707 still advertised (back-pages only) until december 1981 in Japan.
The big Pioneer bible published by Stereo Sound in early 1978 has very little space devoted to the RT-707 : not a diagram, not an exploded view of the drive nor a sample of the design iterations...
It seems a very small batch of black RT-707 was made sometime during the long (export) production run but I have yet to find a catalog image of it.
The design slant of the 701 / 707 was carried on by Akai with the even prettier GX-77, also a besteller. The latter, however, came when open-reel recorders really were out of the picture and was built according to that.
The RT-707 still was clearly on the 1970s side of things : solid.