I've never seen a side mounted Sq D breaker either, don't feel bad. FWIW that's why I'm here.
All single phase Square D load centers (other than the small 6-8 space ones with a horizontal bus) made since this style of main came out utilize this style of main breaker. They are all convertible from main lug to main breaker or vice versa as well. Three phase load centers either have a back fed main for 100 amp or less or use a QBL type breaker as the main, which can be turned upside down if you want to bottom feed the panel and still have proper switch orientation of the handle. This been that way for about same amount of time that they been making the QOM style mains for the single phase panels which has been for around 35 years or so. been some minor changes but basic layouts and dimensions have remained the same. I recently had a bus failing in a load center that was around 30 years old. New replacement panel was rather quick and easy as all I had to do was swap guts from new into old cabinet and everything fit just fine.
1971ish the 125 amp and less main breakers would been one that took up top two spaces on both sides. Today a replacement for it would simply be a QO2100, possibly with a VH suffix for higher interrupt rating if needed/desired. Today's version only utilizes two spaces on one side should still use the original hold down clamp on the foot of the breaker. Supply conductors possibly will not reach as is because the lugs on the original possibly faced up and now will face to the side. Original would have looked like one in image below. I copied from a place that resells this old stuff and it was like $400. Can get today's QO2100 for about $100 at Home Depot.
The 1971 partnumber would have been QO2100TF. The lugs were on top of the breaker not the sides. These went obsolete in the 80s.
Anti-oxidation coatings is maybe done by some, though it is more common with aluminum conductors, nothing that is intended to cause chemical reaction is used.
