(advid) writes:

Surely 300mm is 300mm no matter what make of lens..... ?

It is. At most one of the lenses is marked accurately.
Why not measure the actual lens focal length yourself?

All you need is a pair of objects with clear edges a measured distance
apart. Shoot them from a long way away so both are visible, and the
distance between them is (say) 1/10 to 1/4 of the image width. Then
look at the digital image and determine the distance between the two
objects in pixels.

The distance in pixels multiplied by the pixel pitch of your camera's
sensor tells you the image distance between the two objects. You
already measured the distance between the real objects. Now measure the
distance from the front of the camera lens to the real objects. Given
these three pieces of information, plug them into one of the standard
lens formulas and solve for the lens focal length.

Done carefully, this is very accurate. The focal length of a lens is
*defined* by its on-axis magnification.

Dave