[Mirror Lake's BB]

The failed valve could very well have been a plug valve the "cadilac" (probably appropriate for you FLL) would have been a DeZurik. It may have been less expensive than a gate valve for that size but not sure. I think the break point for a buried plug valve using a simple nut operator is 6 inch making the plug valve cheaper. Over a six, a travelling nut gear box would be required since it is a quarter turn valve, and would drive the price up. Cheaper than a gate, I can't say. Maybe it was and the selection was price driven.

I don't know if the travelling nut gear box was perfected for buried use back then when this installation occurred. And if this valve wasn't periodically exercised over how many decades

Also, the actual wastewater is "sewage"; "sewerage" relates more to the process and equipment.

Just my crappy two cents.