I have attached only a portion of the mooring regulations. But as you can see, the "anchor" you describe it is considered a mooring. A ball is not required, as mentioned in the RSA and seasonal use, as you indicated, also makes it a mooring.
Hope this helps explain it a little better.
270:59 Definitions. – In this subdivision:
I. ""Director'' means the director, division of safety services, department of safety.
II. ""Division'' means the division of safety services, department of safety.
III. ""Mooring'' when used as a noun, means a mooring anchor, or other fixed object or stationary point, with or without a mooring buoy together with attached chains, cables, ropes, and pennants and related equipment used for the purpose of securing watercraft.
IV. ""Mooring anchor'' means any anchor or weight which is designed to rest on the bed or to be buried in the bed of a navigable water body, which is designed to be attached by a chain, cable, rope, or other mechanism to a mooring buoy and which is designed to be left in position permanently or on a seasonal basis.