Quote Originally Posted by wylk View Post
As for your question about round and rectangular doors, I believe round ones showed up roughly 1900 (maybe a little earlier) when manufacturing could easily handle such large pieces at the required precision. But why? What's the advantage? My impression is that it's partly marketing. A big round door is quite impressive. And you can make a case that machining a round door to an airtight fit with its frame is easier.

My impression is that they first showed up mostly in private safe deposit companies.

It was common for larger banks to have two separate vaults. One would use a round door for the safe deposit boxes (once banks got into that business) because they look cool, and a smaller rectangular-door vault for their own cash, securities, and ledger books. At times these were side-by-side and might even share a wall to reduce construction costs. In other cases one vault was built on top of the other (such as One King West in Toronto).

Round doors started fading in popularity around 1960 or so, at least that's my impression.

Getting back to the thread topic, Holmes adapted his side-mount controls to both styles.
When you have a REALLY thick and heavy door, it must have been difficult to machine it accurately enough. With a round door they could mount it on a turntable and make it very accurately round and tapered very accurately too. Maybe marketing , maybe it was easier to make.