OK schwa, I will prepare some. Give me a minute.
EDIT:
So, here are the test projects
https://stash.reaper.fm/38419/test-i...and-index1.RPP
https://stash.reaper.fm/38420/test-index1-only.RPP
Here, when you will make DDPs from each project. Then when importing even to Reaper itself, "index0-and-index1" DDP will import ISRC correctly, but "index1-only" one will import first ISCR for all three tracks = wrong.
EDIT2:
Here are screenshots of INDEX1 project DDP imported into Sonoris, HOFA and DDPTOOLS:
https://stash.reaper.fm/38421/index1...in-Sonoris.png
https://stash.reaper.fm/38422/index1...dp-in-HOFA.png
https://stash.reaper.fm/38423/index1...n-DDPTOOLS.png
EDIT3:
And here there are screenshots of DDPs of those 2 example project re-imported in Reaper v6.05rc1
https://stash.reaper.fm/38424/index1...aper605rc1.png
https://stash.reaper.fm/38425/index0...aper605rc1.png
EDIT4:
Screenshots of DDPs with INDEX1 only markers from Sonoris and HOFA imported to Reaper do look the same
as the Rreaper's own DDP-index1only re-import (= first ISRC duplicated in other tracks).
(everything here tested in 32bit apps and system - Win 7 Pro)