You *can* get a seat upstairs without a reservation. I've done it most times I've gone, but I avoid peak times like I would for any busy place.
I've walked in a few times upstairs for lunch and got a table after a few minutes with no reservation
You might have luck if you go at off times.
You can get a drink at the bar, i have not seen food served at the bar. Reservation is available for both upstairs and downstairs, upstairs is easier to get in and doesn't require a lot of time to book ahead.
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