The two most likely locations I can find for the engineering & experimental shops is first, the old Imperial Wheel plant which was taken over by Buick after Chevrolet and Monroe were finished there. When Buick took over in 1916 the plant had the combined numbers #18 #21 & #22 but would eventually become just #18. The photo at upper left (when you enlarge the photo above) shows a building with arched windows and slanted roof and points me in this direction, but I need further research. The other possible location would be the old Weston-Mott factory #31/#32/#33 because the photo at the lower left shows daylight streaming in from both directions. “I have since found out that (at least some of) the photos are from Weston-Mott”. The east and west wings would provide this only on the second floor. The only other Buick building’s with arched windows in Flint would be the original Kearsley st. plant but Chevrolet was using that structure in 1923 in cooperation with Mason Motors. And also the W.F.Stewart plant #3 located south-east behind Stewart #4. Also the original Buick factory #01 on the inside windows of the east and west wings (the part overlooking the single floor center section). As this photo is from at least 1920 they could have been using both #01 and the old Weston-Mott plant. I lean more towards the east wing of the Weston-Mott factory at this time. I’m thinking maybe both locations.