I have been asked to bring my attention to this post.
This design assembly with the rivets is quite normal and can be found on
almost any motorcycle engine (all be it there should be six rivets).
The small collars provide a controlled clearance to allow the basket to move under the control of the cush drive springs.
The basket and gear assembly have a thin shim washer and Belleville washer to provide lateral pressure.
The rivets are (or should be) machined to 08.8mm diameter to a H7/h7 fit to the collars.
They are of a mild steel type (as with most clutch rivets) and are subject to a controlled Annealing process. This is very important, because upon assembly the rivet heads are formed or 'Peened'. This not only swells the head of the rivet but also swells the shank of the rivet causing it to grip the basket,collar and retaining plate as a sandwich. The head of the rivet after peening should appear domed.
If the rivets are not Annealed correctly, then they will NOT provide the suitable grip, and will simply fall apart.
I have assembled hundreds of different clutches to this design.
When peening with a suitable Dolly, you can 'feel' if there is something wrong with the rivet.
In fact, I have had to change Heat Treatment companies because of erroneous annealing!
Please DO NOT WELD the rivet to the rear retaining plate. The plate is made from a pre heat treated steel and would not provide any safe welded purchase or penetration! You have been warned.
The billet clutches were assembled in England pre 2014. The later units with the pressure die cast baskets were assembled some where else ???
I cannot comment about their quality.
I can tell from the images that the rivets have NOT swelled, because the collars have come out easily from the assembly. Normally you would have to accurately drill out each rivet with a 08.5mm drill to the depth just bellow the rivet head (basket side) , then drift out the rivet with a 08mm diameter pin punch. You should notice only gentle hammer blows to drive the rivet out as the rivet walls collapse inwards. This indicates that it has been assembled correctly and it it doing its job as intended.
The 961 clutch design was sound up to 2014. After that I cannot comment.
However, as stated previously, it should have three pairs of two rivets (Six in total) but doesn't. There is a reason for this

Also, the gearbox design has an inherent design flow. Unfortunately at the designing stage way back before 2008, there was an oversight regarding oil flow to the centre of the clutch. Its not present. Normally oil is fed down the centre of the input shaft to feed the end support bearings and the clutch, either by splash or by pressure of pump. And / or it can be collected by the primary gear, flung outwards and caught by simple weirs in the casting and guided down towards the clutch centre. Non present

Hence the nasty black appearance in the middle to the clutch assembly.