Oxygen Regulator/Flow Limiter

I don't know exactly what this is... but unofficially I would say it is likely either an oxygen regulator that reduces down to a low flow for use with a mask or it could also be a flow limiter regulator used for testing/sampling gas during analysis.  Either way, it does have a standard DIN fitting and comes with din to yoke adaptor.  Sold as is, untested, no guarantees.