I'd guess the R is not a common term. The r should be a complete word such as registered.
In most computers registered ram will work in a system that doesn't require it. The r might also be shorted too far for ecc which is less swappable.
Use a memory configurator to help decide what will work or use the owners manual for better guess.
I doubt you can even find any 2100. They normally substitute a faster version as a choice.
1/3 of highway deaths are caused by drunks. The rest are by people who can't drive any better than a drunk.