And now there are 5. As can be see from these 4 "US" versions and the original post of a "DUS" that make the total of versions to 5. The top picture of a Blue meter unit came from Google Images. The 3 stack are mine. Let's try to figure out the release order and then I will go into what may be the major design differences. I intend to go into the units to service them and I hope to find a date somewhere and then document the circuit differences. The only service manual I could find details the 5th unit: the "DUS" and what I believe is the 4th unit, the Top "US" unit, they both have the eject selector switch, AC outlet selector, voltage selector switch (which becomes a major issue) and DIN audio input/outputs outlined in the service manual. I will go into the differences between the 4th and 5th units later.
I believe that the 1st unit is the middle unit of the stack. You can it has no Pause switch or any kind of switch in center panel.
Next I believe that the 2nd unit is the bottom unit of the stack. It has 2 phone jacks, Noise reduction switch and clear meter bezels. This is important, my unit has green lighting. All of the other units have Green tinted bezels. I have been playing around with multi-color meter lighting for my Akai CR80 and Teac Reel to Reel decks in that I would use Green for playback, flashing Yellow for pause and Red for record. The only RS-858 that has clear meter bezels is the unit that has no pause switch! A transplant may be in order. Adding pause to the Akai is easy in that it is easy to stop the motor, but I have not overcome the rotation inertia of the capstan.
The 3rd unit I believe is the Blue metered units. Notice that the blue metered units have clear bezels and a noise reduction switch. This is interesting because it was always thought that the blue metered units came later, but as seen the blue metered came in the middle of the release schedule. If anyone has a deck with blue meters with a difference front panel layout, please post. Maybe we can get to the "Magnificent 7".
Note: All of this release order may be blown out the window if I or anyone else can come up with a date stamp somewhere in the unit and that date would have to be compared to date stamps in the other units. Maybe if some original purchase receipts were saved?, I know, A long shot.
Now the differences between the 4th and 5th units. From the front panel, it is mainly the phone jacks. Internally from the schematics in the service manual the most glaring difference is that the "DUS" uses a DC motor with it's voltage derived from a rectified DC from the secondary of the power transformer and the "US" units uses an AC motor with it's voltage taking directly from the line or the 115VAC tap. There are so many pros and cons to each method that would take a text book to explain. The advantage of using an AC motor on the primary side is that there is no or little interaction between the voltages on the secondary side being affected from the motor or vice-versa. And if the voltage is within design limits the speed of the motor is not line dependent. The speed of the motor is determined by the AC Line frequency and number of stator poles used. I believe that the fast forward is derived by applying the voltage to a tap in the stator poles so the rotor "thinks" it is getting say 240HZ instead of 60HZ thereby speeding up, a very stressful endeavor. The disadvantage is the same, frequency dependent. The "US" has no 50/60HZ select like the Akai does. This means although you can select 230V for Europe and unless you change the diameter of the motor drive pulley the unit will run slower in Europe. The service manual states 50/60HZ operation, a contradiction.
The advantage for the DC Motor is that as long as the transformer's taps for voltage select are designed properly it will make no difference if the frequency is 50 or 60 hertz. The fast forward on the "DUS" is accomplished by applying the DC voltage to a stator tap making the rotor think there is more DC voltage applied. Since the DC Motor speed is voltage dependent then a great mod would to put in a regulated and adjustable voltage to trim the speed. The disadvantage of the DC Motor is that if unregulated voltage is applied then any line difference can effect speed and and since it is on the secondary of the power transformer there can be interaction from all of the voltages causing say light diming, etc.
The Model Airplane industry has controllable DC motors that could rip the tape out of a dragging cartridge that may be a mod that could be done to put to rest any torque issue.
It has been said that the later units used a better motor, but I am not for sure. The manufacturers design to the limits. One unit may play a fully load tape cartridge easy while a unit in the same batch that is within tolerance may drag.
It would be interesting to correlate the units that drag to what is actually inside. It may be simply a tolerance issue. I will know more as I tear into my units. Any input from others would be great. This mystery may not rise to the Egyptian Kings, but it would be interesting to get the history straight on these units. More later as time goes by.