on-prem 1.5.0 rules are:
1) Take the source replica set's version
2) If that version is exactly A.B.C, look for a folder called mongodb-<platform>-<architecture>-A.B.D, where D is >= C.
3) Else it is considered a custom version and look for a folder called mongodb-<platform><architecture><version>, where <version> ends with the source version. For example, source version 2.4.10-apple-b will match mongodb-linux-x86_64-apple-2.4.10-apple-b.

This only applies when autodownload has been turned off.