Hi, although it is a similar shape to the M68 one in the Whitefriars catalog in Glassobsessed’s link, there look to be significant differences to me:
The M68 one has more of a wasted type neck.
The M68 looks to have a more bulbous body, with narrower base.
The M68 doesn’t look to have an everted rim.
The M68 has a teardrop shape stopper with stem, Glasstime’s is elliptical and of more narrow proportions. The 2 on the stopper and neck are to match the stopper with the bottle, so it should be the original stopper.
I don’t know if this is a Whitefriars decanter or not but I would say not M68. The shape has been around a very long time, McConnell, in the book The Decanter, calls these bottle shapes tapers or broad based tapers, he shows one of a very similar form from 1765. I imagine there would be dozens of manufacturers who have made this shape over the years, including in the 20c.