Probably not news to anyone, but there are counterfeit Cisco SFPs in the market place. It only matters for support purposes – all the SFPs are made to a standard, and it’s just a matter of what the label says past that point. As they say, if it quacks like a duck..
So how can you tell real from fake? Short answer – you can’t easily. If you buy through legit channels, you’re good. If you buy from eBay, there’s a pretty good chance they’re fake. If it’s from AliExpress.. well, I wouldn’t expect real ones.
Going through some old photos, I found this picture from last year from an install job I did – all of these SFPs came directly from Cisco, and they all look different.