If you want to play your 18.4 game again, use steam to download the alpha18.4 game and then load your old save.
If you want to switch often between 18.4 and the newest version I would suggest downloading alpha18.4, then make a copy of the games install directory. This copy can then be used by first starting steam and then starting 7dLauncher of the copy manually (not through steam). Both game versions will use the same settings and the same save game location