[Fungus]

Are all the housing of the um1, um2 & um3x the same size?
 
Or does the size increase with each additional driver added? 

 

[Spyro]

Same size

 

[Fungus]

  Same size

So how can I differentiate between between the um2 rc blue and um3x rc blue since both have blue housing making it difficult to see the additional driver inside.  
 
Reason it I purchased a used pair of um3x rc blue with no box or accessories and finding it difficult to located the third driver. 

 

[Silver]

I think only um3x has back side being painted black, the back side of um2 remains transparent. From official website.
 
http://www.westoneaudio.com/index.php/products/um-series/um-product-archive/um2-rc.html (UM2)
http://www.westoneaudio.com/index.php/products/um-series/um-product-archive/um3x-rc.html (UM3X)
 
Anyone out there feels free to correct me. 

 

[Fungus]

No, you can get both with the black on blue housing. 
 
My UM3X RC 

 
UM2 RC 

 

[Spyro]

  I think only um3x has back side being painted black, the back side of um2 remains transparent. From official website.
 
http://www.westoneaudio.com/index.php/products/um-series/um-product-archive/um2-rc.html (UM2)
http://www.westoneaudio.com/index.php/products/um-series/um-product-archive/um3x-rc.html (UM3X)
 
Anyone out there feels free to correct me. 

 
Yes, you are correct.  The black on blue was a very limited edition and on sale for a very short time a couple years back.