I did a fresh install of Sitefinity 7.3.5600.0 then went to add the Thunder module and received an "unsupported version" error.
I re-installed Thunder and tried again getting the same error.
Is anyone else having this problem? Thoughts on a solution or do I need to wait until a new release of Thunder?
*** UPDATE ***
So there was an update to Thunder released - last night or this morning - to version 184.108.40.206. Installing this update resolved the problem. Many thanks to the Telerik dev team.