75-300 and Extender compatibility?

We've got a customer who wants to hire an Extender to fit to his Canon 75-300. I know that from the point of view of image quality that's going to be a bad-to-awful idea ... but does anyone know whether it's physically possible? Is the rearmost element of the 75-300 recessed sufficiently?

Stewart,

From http://www.velocityrev​iews.com …-telephoto-for-rebel.html

"The Canon t/c is excellent optically but won't mount to the 75-300
without a 12 mm spacer, meaning you lose the ability to focus at
infinity, so you should look at the best models from Kenko"

Bob

Thanks Bob.

I should have guessed you'd know the answer and asked you directly!