The first 6 -7mm of the c'case breather outlet has no thread and if that means it has stripped I've never seen such a machined finish to a stripped thread.
The Comet is a '51 which I understand would have had an ali'bamjo bolt from the get go and that if gorilla force had been used on the bolt it is usual for the bolt to shear before the case threads are damaged. Although this does not preclude ham fisted abuse of the thread in the past or a steel banjo bolt having been fitted and chewed the thread. But again, the first 6-7mm of the hole looks like a machined finish.
So having considered making up a longer bolt, the threads beyond the first 6-7mm appearing to be in unmolested condition and the bolt picks up on them quite nicely but without the banjo union, I went for the expedient solution; removed the washers and used a smear of sealer instead: Result, enough thread engaged and bolt tightened down quite firmly.
After a 80 mile bimble into Kent the motor and chassis are freer of oil mist/leaks than ever, next step will be to do 'O' ring mod at Burman splined gear shift.