As ol' workbuddy used to say, ' are you pickin' up what I'm putting down' and I'd say 'yep it's stuck to mah boot'....but I'm pickin' up what yo're puttin' down Swarfo....
.........SRM sell the same spring and ball for A10 and A65.....
I've attached a pic of the bits I bought from SRM a while ago- maybe they changed their ideas
, 2 identical balls & 2 springs; one is A10 and the other is A50/65.....FYI, The plug diameter is 0.365"/ 9.29mm, just under 3/8"...
Not 100% certain which is which, but I think the A10 spring is the shorter one with less winds, if you really want to delve further, there's heaps of 'coil spring rate calculators' on the web
When I first heard of this A65 concept, I figured there must be a way that the ball is contained in the case by some push-in upstream seat kinda setup, but apparently not so...
And It just occurred to me
that in order to reduce loss of pressure,instead of a ball as a valve, maybe a Carby type float-needle valve may work better....
...
(unrelated but, in concept- I have a floor jack, that had a leaky bleed-down valve and as bits are un-obtainable, the only way I could fix it was to put a bit of cut-off Allen key with a groove around it in there so oil would still flow- yea had to be there )