But if they were the correct period alloys for that car and a very common option, nobody would be able to say how the car was delivered to you, particularly if you bought it 2nd hand. Bottom line in this type of situation is that Insurance Co decided to charge more because it could and because you asked. In fact the very fact that someone wants to make a change to a car is what often flags up increased premiums, as 'modifiers' are seen as fundamentally more risky, irrespective of vehicle concerned.
Insurance co see it as a 'modification', which can be assessed as higher risk, hence higher premium.
A few years ago, one of my learners had bought herself a 106, very tidy, with OE alloys. Insurance co aksed about mods, listed a few including the question of alloys. According to their records, it was not supplied with alloys when new, even though they were an option for the first owner. Difference I premium then was over double. She was devastated.
I suggested the seller might well have the original steels in his shed/garage/loft and be pleased to dispose of them. No market for steels for 106/Saxo. The following week, it sat there on steels with new tyres. Seller had given the wheels, a local independent tyre shop had fitted new budget tyres, sold the alloys for her, and given her some money back!
Another student, after passing his test, added alloys, paid the extra premium, then added bodykit, factory approved suspension kit, then had some factory approveed engine work too, with a special exhaust developed for that. All was factory approved for Corsa. Each mod added more premium, until one point where the premium started to drop instead. There comes a time where all the mods were viewed differently, with all that spent, the threshold had been passed from 'idiot' to 'proud enthusisast'.
That was over 10 years ago, he still has the car, now as an occasional use toy.