Because it is just stamped Sheffield and not Sheffield England it predates 1891. Thomas Turner & Co was established in 1802. I'd guess closer to 1891 than 1802. Nice work BTW.
Yes, pre 1837 because of the inscription of "His" majesty. Looking at the design, it's not likely to be George 3rd so it's either George 4th or William 4th putting it between 1820 and 1837.