Now that is interesting. I wouldn't have expected Buicks and Pontiacs being built on the same production line. Or if not on the same line, in the same building. Said it before, I love production line shots after 44 years in the motor industry. Mart.
Back in '59, most Buicks were built in Flint MI, most Pontiacs in Pontiac MI. I lived about 5 miles from the GM Proving Grounds near Milford Michigan and this photo might have been taken there or at the GM Technical Center in Sterling Heights Michigan. I worked across the road from the Tech Center in later years. There were hundreds of cars & trucks at both facilities.
Sold in France , 2014 https://www.autoevolution.com/news/...l-air-up-for-auction-photo-gallery-74933.html
Flint was of course Buick-only in this era (the 'home plant'). Buick-Olds-Pontiac plants were Linden, Kansas City, Wilmington, Atlanta, Framingham, & Arlington. The production line pic was from one of those 6. But Flint wasn't the busiest- according to an August '58 Buick production log, Linden was #1, Atl #2, KC #3, then Flint. The other 4 were basically tied for 5th, about 25% slower than Flint. Linden was running about 225% above Flint.