Wow, that's a cool look, though it looks like it's going to be a pain...

Remove one to measure! I've definitely flubbed measurements by 3 mm when I was eyeballing them/trying to do them without disassembly.

If they actually are 19 mm, you might evaluate if you'll get enough purchase on the nipple if you simply use the 16 mm nipples. I've seen vintage wheels (an old Schwinn S-6 wheel in particular) that give about 5 mm of dead space between where the rim ends and the spoke wrench begins. It's hard to tell from your picture, but this may be true to some extent about your wheels. Newer wheels, by contrast, seem to have the spoke wrench nearly flush with the rim. Even if they come up short, you could do other monkey business (like filing down the business end of the spoke wrench to decrease the clearance needed). If you doctor a cheap spoke wrench (like a Spokey), and if the wheels are worth a lot to you, that might be worthwhile.

Best of luck with those weird, cool wheels!