This is definitely a new one for me. There are 2 versions of the girder spring top mount but you have the same type on both your bikes which has a flat top surface above the fork spring. The earlier type has a more conical surface. It took me years to spot the difference between the two. That difference is the angle of the "slots" which take the top of the spring. The early type has the slots tapered the same way as the spring coils. The later type has the slots tapered the opposite way, presumably to grip the coils of the spring better. Clearly this was seen as a problem since the drawing for the spring instructs you to fit the spring to the top mounting before enamelling.
Given this evidence of problems and developments in this area I can only conclude that your extra fitting is another attempt to solve the problem.
The problem, in my experience, is that the top mounting comes loose in the taper fitting in the top of the fork girder. No way could I keep the one on my RH tight so I used to run it slack which it survived happily for many years. Then the threads stripped in the top mounting so I had to go up a thread size (but the same hex size on the bolt head for appearance sake.) I still run it fairly loose and it gives no problems. Despite many bumps ridden over, the spring top mount has yet to become detached and if I haven't done it by now, I don't think it is going to happen. So perhaps the problem is one which causes fairly instant concern, but in the end, doesn't matter.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser.