Is there a a description of how to stop this happening?

yes

dont reverse at full speed
dont lift off while reversing
dip the clutch before braking while reversing
peen the selector ring if you intend to use reverse

Mike Phelan did a "what happens" article think you may find it listed on some posts here as well as 2cvstuff

Don't drive down a dead end

Thanks for all replies and pics Ken,think ill push mine backwards from now on
Think you have mistaken this car for another James,old guy had this one for years,woun"t sell for love nor money,even reluctant to let me have a look at it

Sean,
maybe add 'don't reverse up a steep slope'?
That induces the same loading inside the gearbox as the manoeuvres you've listed.

I also think that the tendency for the nuts on the rear of the mainshaft and pinion shaft to slacken off on later gearboxes is part of this problem.
Even if the nut is only a part turn loose, that will allow the mainshaft to move forwards under engine braking in reverse, etc., etc.
Play in the mainshaft rear bearing has the same effect, but most folk take heed of the noise resulting from that fault.

ken.

Definatley! a loose top rear nut or a slack bearing race, and its the first to go when the oil is low, I reckon is the start of the chain of events that overcomes the weak staking. The bevel cut on the gear produces thrust on the shaft and rapid changes of load can have the shafts clonking back and forth easily seen with the lind off a box with loose shaft nuts or a run top rear bearing.

S