Some more useless markings are 2Rx8, 2Rx16, 2Rx64. All can be, and usually are, both low and high density.
Kingston makes almost all high-density RAM now. They do have some low density like 128x8 sticks for DDR2 667 1x2GB still being made in valueram. These will have 16 chips on them and usually be marked 'compatible ram'. Kingston IMO is one of the best manufacturers out there, I've been using them since the 90s, but they also have this problem.
Crucial doesn't even make low density anymore but shows some of there high density RAM sticks as compatible with all the low density only systems. I actually commented on "compatible" sticks for low density systems on their site, and they censored all my comments. This could only be cause it'd affect sales.
Samsung and a couple other makers still do a lot of low density, but it's mostly for OEM. You can find them online new still though.