The help output says:

You don't have to prefix with v. E.g. -i 4.9 is the same as -i v4.9.

However the output of ubuntu-mainline-kernel.sh -r 4.9 ends with:

v4.14.98        v4.14.99        v5.4.9

Omitting the v enables substring matches which can lead to install making the wrong choice.

It seems better to treat 4.9 as v4.9., internally.