This is one of those changes that Porsche claimed occurred
with the model year 72 to 73. I have seen 72 models with
the dimple and 73 without. Certainly by mid-year 73 all the
911s had this change. (Dimple red circle, bolt head green arrow.)
04a.jpg
"
© Dr. Ing. h,c, F. Porsche A.G.
If you mod an earlier torsion bar tube for clearance, you
should carefully inspect the interior of the tube for rust
damage. Under no circumstances should the torsion bar
ever be able to contact the tube even slightly will lead
to the torsion bar failing.
There is an even more important change that occurred in
this area with the '74± model year. Porsche added two
small reinforcing pieces between the torsion bar tube
and the aft end of the tunnel (yellow).
IMG TorsionBarTube03a.jpg
These transfer the load from the trailing arm (banana)
to the chassis without bending the torsion bar tube.
This is a
very desirable addition. Someone
should manufacture an aftermarket part as it is only
available from Porsche as part of a replacement torsion
bar tube.
Best,
Grady