Show Image
(http://i.imgur.com/TJ5eN94.jpg)
This is my 1391401 Model M with all Unicomp keycaps. The blacks are single piece, the purples are two pieces. The covers on the two-pieces are compatible with the keys from my old Model M.
The Unicomp two-pieces feel very slightly heaver to press down, but I haven't made any measurements. I couldn't even tell until I looked out for it to make this post.
Switching out the interior piece of one of the unicomp purple keys with an original from the model m resulted in it feeling like the lighter, single-key black unicomp keys.
Hope that helps!
Welcome to Geekhack!
Still wondering whether it depends on the colour as to whether you get one or two piece caps, or the batch. Doesn't really make any difference as they sell you both of the two piece ones anyway.