Difference between RainSoft EC4 Models

I have some Rainsoft compatibility questions that I am hoping you can address.

To set the stage, here are the Rainsoft items I have - transcribed from the labels on the units, and a bit of background info ---
• Unit 1
Model: EC4 100 CT 110V PT OD
Part No: 31325
• Unit 2
Model: EC4 100 CT 110V WALL OD
Part No:31259
• Unit 3
Model: EC4 OXYTECH 150 MM 110 RT OD
Part No:31512
This unit is larger/taller than unit 1 and 2
• Unit 4
Model: NO LABEL
Part No:NO LABEL
This unit is larger/taller than unit 1 and 2
The face plate under the plastic door says, "RainSoft-Taking the Worry Out of Water ...Since 1953 -- OxyTech--Chemical-Free
Iron, Sulpher and Manganese Reduction--EC4 Technology.
• Unit 1 has been in service for a while in our home but the display is unreadable - it it flashes and some segments do not light up such that it is unreadable about 50% of the time
• Without realizing that there are different EC4 models, I purchased units 2, 3, and 4 used at a steep discount with the intent of swapping out one of the three boards in the control unit with my unit 1 that has the unreadable display
• When I made the used equipment purchase, I was told they all work but they have not been in service for about a year
• It wasn't until after I got home after making the used equipment purchase that I realized they are different model and part numbers - I will not make this mistake again on a used purchase

My questions:
• Are the EC4 boards in units 2, 3 or 4 compatible with unit 1 such that I could swap the control board to fix the crazy display - even though they are different model numbers?
• What is the difference between these four units?
• Any idea why unit 4 does not have a serial/model number label on it? Is this normal

Thanks in advance for your time!!!