dont think there is any seal mate, the reg should be tight on the bottle but by adding air into the bottle the reg will tighten on the bottle, or so me believes
There should be an O ring on the reg, there was on all the ones I have done. It should be tightened with good grips. I always go as tight as I can.
Perhaps the seller used to sell the bottles and regs as individual parts and just screwed them together to send.
Nige