Good spoon.
The hallmark is for sure London assay, 1798 which means the maker was
not Naphtali Hart. He was in partnership with Duncan Urquhart at the time and they registered a series of marks as 'DU above NH' in 1791, 1795, 1801 and 1812, extending their wares each time they registered (starting as “bucklemakers” and with their final mark as “plateworkers”. Hart did not have a solo mark registered under his own name (as NH) until April 1812.
There are rare instances on English silver of the assayer either inadvertently selecting the wrong punch (correct letter, but wrong style) and also instances where a punch was broken and a lazy assayer used the nearest available. As well as happening only rarely, there are other reasons why I’m sure that wasn’t the case here (ie it's not the lower case 'c' which would be for 1818).
I think the mark you’re reading as ‘NH’ is actually ‘HN’. There were two makers using an 'HN' mark in 1798: Henry Nutting (registered in April 1796) and Hannah Northcote (registered in June 1798). The similarity of their marks has created uncertainty about attributing pieces, but on balance I think this will be by Northcote, since she had been married to a spoonmaker before registering her own mark after his death. Nutting was more famed for larger prestige pieces such as tea/coffee pots, salvers, jugs and such… but it could be either.
In the picture below from a hallmarking website, the Nutting/Northcote mark is shown
upside down so that it appears to read ‘NH’ (even expert websites get it wrong sometimes):
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You can confirm this by looking at the serifs on the N letters and comparing to your own as shown below. The final limb of the N has no serif at the bottom when it’s the right way up. Note your mark also has a stop between the two letters, but neither of these marks have that. These kinds of minor variations are not uncommon on early marks.
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What you have is generally known as a “stuffing spoon” and used for removing the stuffing from turkeys or game birds at the table, but also commonly used as a buffet serving spoon. They usually measure 12-13 inches in length.
The bird engraving will be a family crest, but I think it more likely to be a falcon/hawk rather than an eagle. It’s not Arkwright since the bird isn’t ‘rising’, and I haven’t spent much time looking, but I think it may be Hadley… for which the crest on the armorial is a falcon holding a buckle in its beak, as shown atop the shield here:
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