This is a Swan Leverless 4461, probably the largest Swan made in the post-war period.
This was an expensive, high-prestige pen. Not all that many were made. I don't believe it would qualify as rare today but it is certainly uncommon and highly sought after.
Like large, high cost pens made today, the 4461 was not bought solely for writing. Bearing such a large piece of gold, it is as much statement as fountain pen.
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