Just a few sets that are still in the works are actually compatible with 1800 keyboards, this is one of them, if it hits production first, it would be the first set with full 1800 compatibility, because, it includes a 6u space bar, that some 1800 need and also 7u space bar that some other 1800 models need.
Edit: Here is a sample of 1800 models; there are two main types, the ISO (6u) and the ANSI (7u), and they are partially covered, there are a couple of 1u mods missing Alt and Ctrl fully. Pictures are not mine, I borrowed them from the Post your 1800... in the media sub forum.
Ideus, after looking at 1800 boards for a while, I think official Cherry-produced boards only comes in two variants, older 1800 with 1.5u mods and 7u spacebar, no winkey or menu. And newer 186x boards that has 7x1u mods (all the ones you would expect from a modern keyboard) and a 6u spacebar. I'm fairly sure I've seen the newer 186x variant in both ISO and ANSI (likewise for the older 1800 boards).
The boards with the other layout such as 6x1u mods with 7u spacebar looks unfamiliar. I'm unsure if Cherry ever made boards with those layouts, but I do know that aftermarket 1800 PCBs like Leeku1800 does support those.
I believe this set will fully cover the older style WKL 1800 board with appropriate legends, but not the newer 186x style ones (unless you're willing to have the legends not match the keys, in which case it's possible, since you do have enough R4 1u keys: 2x1u Winkey, 2x Diamonds, Ctrl and Alt, and the Fn key).