Thanks Guys, please keep feeding the boiler!
As you wish
(though I suspect you may come to regret it)
I have not yet found a viable POD to butterfly an earlier proximity fuse. If someone has information as to how the VT fuse could have been produced earlier, i'm all ears!!
I think the key to your dilemma is that you have actually asked
two separate questions and have been looking for only
one answer.
To clarify, IMHO the PAM should first be looking for a way to create
any effective anti-aircraft weapon with a proximity fuse (whatever the weapon format)
and only secondarily finding a way to use proximity fuses in shells fired from the most common conventional heavy/medium AA guns.
(For the time being the fact that proximity fuses are useful in land warfare can be ignored,. It the Perfect
Air Ministry after all )
Looked at from that angle it's not such a great step to imagine that iTTL the UP rocket systems to be fitted with the Thyratron based radar fuses
(rather than the snagging wire systems used iOTL that proved such a failure.)
Later Edit: just found that the initial intention for the 3" variant was not the wires but an experimental optical proximity fuze!
You would probably have to accelerate the development of both launchers
and radar fuse a bit, but that is not too large a jump
(especially as Lindemann has less impact iTTL)
Aside: it may be better to limit the variants of the rocket to only 7" and 3" variants (Y and Z batteries?)
Not so unlikely given the new fuze.
The low acceleration and lack of spin from the rockets mean that the technology is producible in Britain
and given the low volumes needed for these applications, the Americans need not be involved in 1940
(which matches well with TTL Tizard Mission parameters)
In truth, I doubt that enough UP batteries would available to make much difference in the BoB
but they very well might make the later stages of the Blitz harder for the Luftwaffe
Fortunately, being used only as supplementary AA at Sea, in Britain (and maybe few bases like Malta) security is less of an issue.
The PAM would not forget the second stage and so development, including the cooperation in Canada, would still go on during the Winter of 40/41.
The new valve and battery give the British all the design elements of a gun-fired proximity shell ... but probably insufficient production capacity.
The British can build and test a few batches in spring 41.
At that point, the British could approach the Americans with a proven design that is "patented" (rather than a "possible" concept that can be "improved").
The result would be a production contract in August 41 on favourable terms in return for allowing the US to build the same mechanism in their calibres.
This would avoid most of the nearly years delay while the US reverse-engineered the British design and repeated their tests.
Hey presto, the RN get 4.5" & the Army 3.7" VT in early 42 and the USN 5" VT in Mid 42.
From that point, the technical track is as OTL with US firms making changes to ease production
and US scientists leading the way in miniaturisation so that the Bofors 40mm can have VT too, perhaps even a little earlier than OTL.
OTL multiple 3" version in 1941