Paolo,
I don't know about early MK1's, but in "la Ronde Infernale" or one of those other films, you can see the mechanics remove the spare from under the nostril panel, bounce it off the ground and reinstall it under the panel during a pit stop. I may be confusing the years as it might be footage of '66 with a MKII but it's in a clip somewhere. Check
THIS THREAD for the '69links.
This is hearsay on my part but I think I remember reading somewhere on the forum here that procedure was required by the FIA.
The MKIIb's had their spares mounted in the rear adjacent to the transmission with the FIA luggage space on the opposite side. The spares are clearly visible in photos showing the rear panel.
THIS is an excellent site for pictures.