Well, Rapier is a manual CLOS system with a basic surveillance radar and originally daylight only; Blindfire capability came later. Sea Wolf is a fully automatic system, it is also 'navalised' as it has to work in rough seas.
There was also need to engage smaller targets (antiship missiles from their frontal aspect, for example), which drove the naval solution that became Sea Wolf to a larger proximity-fuzed warhead instead of Rapier's small contact-fuzed lethality enhancer.
Well the two pursued solutions to their respective needs seperately because their needs somewhat differed.
The Army concern was size, weight and, mobility was critical to them, while the Navy emphasised effectiveness against Soviet anti-ship missiles.
Only briefly did they converge with Mauler, only to seperate again once that failed.
Had Mauler entered service with the US Army and USN then it's plausible that the British Army and Royal Navy would follow suit.
It is arguable that they could have found a common solution. But at some degreeof compromise for each.
The success of SeaCat/TigreCat did hang over them.
I understand that some degree of compromise would be required from both services to create a common SAM. However, I believe that they could both start with a somewhat simplified system, which could be "navalised" to work in rough seas. To that could be added equivalent to Blindfire radar capability to allow the system to work at night. From that, could be developed a much more capable system, with, if necessary, larger missiles with larger warheads.
This comes to the problem that the RN had already been working on short range SAM systems through the 50's.
Hence Popsy, Mopsy and Orange Nell.
And in retrospect the earlier Brakemine GAP might have set them up better for the future.
But in context I'll suggest that PT.428 could be achieved for both services by the late 60's.