Lets see
1st - 87UW - White 87U with US layout
2nd - 87UB - Black 87U with US layout
3rd - White 87UKW or 87UKW55 with Korean custom keys. Hard to get over here in the US but you can if you are willing to pay out the nose for them. The keyboard with out the custom keys is around $330 before shipping. Not sure of the cost on the keys it has been awhile since I've looked them up.
4th - 91UBY - Basically the 91UB with yellow keys. I would not be surprised if Realforce offers this option for the 87U. But not to sure I really like the yellow keys they are just a little to bold for my tastes.
The only new one in your line up is the 91UBY but Realforce also released some other derivatives of the 91U at the same time and a couple of 108U updates as well. Essentially all they did was take the current 91U and added some yellow keys for the 91UBY the others they just changed the key weighting a uniform 55g board for both the 91U and the 108U and a uniform 30g of both as well. The 30g boards are not really new as they have been OEM products for some time.
EDIT: Forgot that this thread was started by one of our new members which means he most likely does not know where to see all these boards. Here is a link to
diatec keyboard section the Realforce boards and HHKB Pro boards are about halfway down the page. They are not hard to find just look for Realforce and you will find them. Clicking on any of the boards will bring up that keyboards details along with a bunch of better pictures of the board.