Original ones have a different profile to repro ones. Repro ones fit over the slam panel and have a more pronounced peak to the middle, original ones fit with the contour of the front panel and are more gentle in their peak. The very very early ones (could be as early as only the first few months production, so up to end of June 59) are chromed brass but we don't have any idea when they changed, it's not in the parts books. Later are chromed steel then stainless steel.