Trying to identify this air flow tester

I have run across one of these before in my search for CFM airflow testing. I found one very much like it in France, but with a different number of rings. There is one currently on eBay which is identical aside from the gauge. Does anyone have any information on this or a set of instructions? https://preview.redd.it/bsfyw398z0de1.jpg?width=3024&format=pjpg&auto=webp&s=b639ede1b2189ba2680a37ebfc827163c31816ba [Airablo link](https://www.airablo.com/products/testeur-a-vacuum-airablo-271-cfm?shpxid=e416b32f-e74f-4553-94c8-44cccd647974) [eBay link](https://www.ebay.com/itm/115266389516?_trkparms=amclksrc%3DITM%26aid%3D777008%26algo%3DPERSONAL.TOPIC%26ao%3D1%26asc%3D279071%26meid%3D1c264673540649848d5882c2ac4a8dcb%26pid%3D101952%26rk%3D1%26rkt%3D1%26mehot%3Dnone%26itm%3D115266389516%26pmt%3D0%26noa%3D1%26pg%3D4375194%26algv%3DWatchlistVariantWithMLR_BP&_trksid=p4375194.c101952.m162921&_trkparms=parentrq%3A66a7ff2e1940a8dde3e3b329ffffcf8e%7Cpageci%3A15eec9ac-d2bc-11ef-bd3f-ce8f59f0065d%7Ciid%3A1%7Cvlpname%3Avlp_homepage)