Actually, I found Signature Plastics key profile to be confusing and neither WASD or Das follow the same terminology.
I am hoping Filco doesnt either or ordering WASD custom keys for a Filco will be problematic.
Someone had bought a Das Ultimate years ago [no printing on the keys] and had taken them all off to wash them and then couldnt figure out which ones went in which row as he had never sorted them assuming they were all interchangeable.
He asked someone at DasKeyboard and she replied with a side picture of the 6 rows on a standard keyboard with numbers on each one to indicate which rows were "dupes".
I forgot my USB stick with my Opera bookmarks at work otherwise I would have posted the link here but I still remember it.
Basically, from
bottom row to top row, the keys were numbered as 1,1,2,3,4,4 meaning the top two and the bottom two rows are identical while the rest of them are different.
I have two Das's [Brown & Blue] and have ordered replacement sets from WASD already without issues
The WASD order page for custom keycaps uses the same terminology
http://www.wasdkeyboards.com/index.php/products/individual-keycaps.htmlHTH