Side panel steel vs Fiberglass

My 74 Interstate came with fibreglass covers but was able to get steel replacements, 72 Roadster has steel panels. Not exactly same as yours so wait for other replies.

My '71 Roadster has fibreglass sidecovers

My Sept '73 Mk 1A 850 Interstate has fibreglass panels (original). My '76 Mk 3 850 Interstate has steel panels. I'd suggest that '73 year Roadsters and Interstates all had fibreglass panels.

Andover Norton parts diagrams list both from 1973 and 1974, I expect it was a case of supply what was advailable much the same as now.

January 1974 850 Roadster with steel panels.

The last batch of 850 Roadsters and Interstates(1977) had steel panels. It was probably a supply issue that favoured the fibreglass panels as noted by otherssince before and after, steel seems more common.