Thanks for all responses. It sure looks lika an intentional modifcation, but the real reason to why this was made, I guess we can only speculate.

I will take both swingarms to a welder and see what the difference in cost it would be to restore the modified swingarm and repair the original (missing bobbin lug)

I also found another interesting mod to the linkage plates. Has someone tried this modification (see picture)? I thought that the bikes in Sweden were all fittet with Öhlins rear shock by Honda Sweden wen they were brought here for racing purposes, but this frame I bought with a SP rear shock. To change the ride height I guess this modification was a better solution than just adding preload to the spring.
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MC21, GP-Replica (Daijiro Kato, Telefonica Movistar Team 2001)