Radio Shack had some very high quality components before the 1990's and I know that they were somewhat early on the importing of Japanese HiFi. Who custom made their components for them? Their stuff definitely appeared to be unique to their establishment unlike JcPenney, when they sold good quality house brand components it was usually NEC or Technics with no more than a different brand name on it (Curtis Mathes and General Electric also rebranded NEC and Technics for high fidelity item in the early 80's). I think I heard that back in the 60's, there were some Realistic receivers that were re-badged Pioneers before the Pioneer name became well known here.
The information was never really revealed..Indeed there were times as employees we would open up a previously repaired receiver to see if we could find any identifying logos.
I was a huge audio-file as a kid. My first ever save and purchase at 11 was for my own Technics brand receiver... Before I purchased it I hung out at tech-hi-fi, Tweeter, and of course Radio Shack listening to everything.
My First Store Manager at Radio Shack was a consummate advocate of the "Sell a Stereo via the play it loud with good tunes" Method. I later employed the same methodology to sell Home theater systems via judicious use of Video Tapes cues to specific scenes...Jurasic Park, Star Trek The Undiscovered Country.
But I digress, My Manger and I used to talk good equipment and we theorized based on our own market comparison research that Onkyo/Pioneer made the 90's Optimus brands. An Optimous receiver and Onkyo receiver being repaired during the launch of the "repair shop" at radio shack confirmed my suspicions as the Volume controls were almost to alike to discount. (The CD magazines I think were interchangeable between optimus and Pioneer CD players)
My First manger who I kept in touch with all through my Radio Shack career by calling whatever store he moved to I brought it up our conversation. We then theorized that earlier Higher end Realistic Brand receivers were made by Nikko Electronics in Japan.. Your question inspired me to wast some work time while waiting for a server to reboot and google and wiki search with not much results however pictures of 80's realistic and Nikko stuff look eerily similar especialy in the cassette decks...
Some of the suppliers I can remember were: Foster Electric (stereo receivers, amps, tuners), General Research of Electronics or GRE (scanning receivers, communications receivers including the DX-150 series and SX-190 & AX-190), Funai Electric (portable radios, table radios), Hitachi (stereo receivers), Pioneer (Allied stereo receivers), Utah (Americana speakers, replacement speakers), Nippon Columbia (the 909 and 999 tape recorders), BSR (record changers), Miracord (record changers), Trio (vacuum tube tuners, amps and stereo receivers). There were many others like Electro-Voice and University Speakers depending on how far back you want to go.