How do I tell where my G4 came from?

I have a G4 that I thought was SIM free, bought from CEX - I'm in the UK and it has no carrier splash screen, no carrier apps or random antivirus trials. To all intents and purposes it is stock. However today I received an update to software version v10G-234-20. This is not Marshmallow and it ran in at 343.19MB. I've seen no mention of it being released anywhere, which leads me to assume that it is an update from at least a month ago. The model number is LG-H815. I check for updates very regularly so this update certainly hadn't been available for my device a couple of days ago.

So my question is, why is this update only being pushed through now? And does it mean my device will get Marshmallow incredibly slowly too?

Thanks in advance, and apologies if this has already been asked/answered before, but I couldn't see it anywhere.