Yes - that's exactly what it is.
Their origins go back quite a long way. Wooden mitre planes were made in the 18th century, and many had a block of boxwood set vertically at the front of the mouth, in such a way that in planing the sole true to compensate for wear, the trimmed block kept the mouth tight. Their purpose was the trimming of end grain on large mitres, commonly used on furniture and architectural work of the time, and some had an iron-shod side to run on a shooting board. Finding originals in good condition is rather rare; they were always a 'high-end' craftsman's plane.
At some point in the late 18th century, metal mitre planes came into being, usually (according to Holtzappfel) having a bed angle of about 25 degrees, and sharpened to about 35 degrees, giving an effective pitch of about 60 degrees. R.A.Salaman (Dictionary of Woodworking Tools) says that mitre planes were used for shooting mitres of all kinds, but with their rigid beds and narrow mouths, were valued for their ability to perform particularly fine work on end grain and hardwoods. Given some of the fine (but awkward to work) timbers used in cabinet work in the late 18th and 19th centuries, that would be of value to some craftsmen.
They pre-dated (and possibly inspired) the infill planes developed later by Spiers and Norris (and others) in the mid 19th century, and pre-dated Leonard Bailey's new-fangled monstrosities by the best part of a century, so good examples excite collectors - hence the price of the example above!