I've been working on commercial natural gas heaters since I was 15. Nearly all of it was plumbed in the 1960's and 70's, with simple high pressure vented regulators plumbed straight into 250,000 or 400,000 btu heating units.

We just bought another OLD facility, that was started in 1929 and probably had natural gas added sometime in the early 1970's, but was partially upgraded in the last 10 years. Some of it had regular 2.5psi vented regulators plumbed straight into the heaters inside the building. Some of it has the 2.5psi vented regulator (inside the building as well) with a second "vent limiting device" set at 2.5 psi with a 7"-11" wc rating.

Has the code changed to  require a second the second regulator and what purpose does it serve?

And why in the hell would you put vented regulators inside a building? These are just greenhouses, but they'll still hold a ton of gas if they vent inside. I've seen this across 3 facilities, all built at different times by different people. Looks like we are going to have to plumb these things to vent outside.

maybe it was propane before natural gas.
They had individual regs on each device. When the conversion happened, they installed the obligatory "main" regulator.
Just thinking outloud.

They probably considered the greenhouses outbuildings. Not "airtight" hence venting regulators. Or maybe Bubba installed them.