shut off valve connected to pressure regulator (out) - why?

I bought a Smiths micro torch at auction that was bundled with several regulators. This auction was an R&D labratory and it appears that numerous compressed gasses were used. One regulator was obviously the acetyline regulator (and came with flashback arrestor / check valve attached). One regulator Union Carbide (Linde) UPD 3-150 initially appeared to be the oxygen regulator - 4000 and 200 psi on the gages. I could not find any tech specs on the gage - just a lot of listings on eBay. But nowhere on the regulator is there any notation that the reg is for oxygen use. This regulator has a shut off valve on the regulated (low pressure). See image. Why this valve?

Second question - could I use the gages on another oxygen regulator? I've got a spare O2 regulator with dinged up gages. Nowhere on the gauge do I see a "use no oil" note.

Images for the initial post.

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Shut off on low pressure side means the regulator was feeding something that didn't like incoming gas at an incorrect pressure. If its out of a lab no tellings what was hanging on it.

For example we had similar set-ups in the white spot nitrogen supplies to Joule-Thomson coolers. Running at too low a pressure, however briefly, promoted icing and general misbehaviour. Back in the day J-T coolers were temperamental little so-and-sos. If you wanted liquid nitrogen reliably and quickly in your detector dewar you had to learn to co-operate with the their little quirks.

Clive

We have shut off valves on virtually all of the gas cylinders in our labs, probably hundreds, maybe thousands of them. They are mostly used for analytical instrumentation or experiments and we shut them off at those valves when they are not in use. That way we don't have to worry about setting the proper outlet pressure or back filtration of gases into the lines when the cylinders are put back into service.

Thank you both for the replys. Makes sense now that you explained it. My limited regulator use is with oxy/acet rigs and for that I was taught to back off on the regulator pressure adjustment after each day's use.