Grohe faucet supply line installation issue

I am trying to install a Grohe Europlus faucet. The faucet's supply lines are a little short so I plan to lengthen them using 3/8" compression male/female ended braided extensions. I noticed that the Grohe supply lines don't have any rubber washer or "O" ring in them. Does this mean they are meant to just have a metal-to-metal compression fit? I'm worried that would leak.
I've attached a some photos of the Grohe supply line's compression fitting. You can see that there's no rubber seal around the nut. Does anyone know how this should be installed? Thanks in advance for any help that can be provided.

It seals to the compression fitting using an "integral bevel" rather than the one on a compression ferrule. If anything it is MORE reliable than a rubber or brass ferrule.

Thanks for your reply. Do I just tighten the Grohe fitting onto the stainless steel compression fitting of the extension line? Attached is photo of the Grohe supply line fitting and the extension line. I've seen metal on metal sealing for Swagelock fittings on scientific equipment, but this is something new for me with home plumbing.