The shim was a makeshift repair, but we can't know the cause.

Possibly the head tube was reamed to accept a larger diameter headset (though I never heard of any in this size), and so needed to be shimmed to accept a 30.2 (nominal) headset. This is the most likely scenario if both top and bottom were the same size. (note: your Magistroni cups are smaller than current standard)

It's also possible that a prior mechanic used an adjustable reamer and simply screwed up.

If only the bottom cup is shimmed, the head tube could have become bellmouthed over time and this was a correction.

Shimming headsets isn't a standard practice, but isn't that rare either. Often it's a choice between using a shim or junking the frame, so don't read more into it than there is.

You have a few choices going froward depending on your skill and access to tools. You can simply continue as is, or build up the ID of the head tube with silver solder, or jB Weld, then ream it to the correct size, 30.15 for an ISO headset. You might also try your luck with a JIS (30.2 nominal) headset which might work as is or be close enough for a bearing retainer grade of Loctite to do the job.