I have a set of rims from an '88 or '89 ix that are 15" BMW wheels with no center caps. Research for original specifications shows only 14 inch wheels on that model. Were the 15" wheels an option?
I would've thought not, realoem.com appears to indicate otherwise - checked a random production month, 10/88 shows a 7x15 wheel in addition to the other sizes. https://www.realoem.com/bmw/enUS/showparts?id=1203-USA-10-1988-E30-BMW-325ix&diagId=36_1392