The authenticity of this sonata is not in question; it's Haydn through and through. Although Georg Feder has placed this sonata amongst the "early" group of works, other editors (such as Landon) tend to place it a little bit later. The accomplished nature of all three movements (particularly the Finale) seem to imply a somewhat later date for this work than the previous four sonatas, at least to my mind. However, the work could not be very much later than its companion pieces, given that it shares some distinct characteristics with the previous sonatas in the series.
First Movement: Allegro moderato
I would consider this to be the finest first movement in the sonatas so far encountered in the series. It features a wide variety of rhythmic patterns (which supports a leisurely tempo so as to make sure all the distinctions are heard clearly), and is carefully structured. However, the sonata form elements are not particularly clear: neither the secondary nor closing themes are strongly delineated. A minor-key, chromatic theme may or may not be considered the secondary theme (it probably is), and gives way quickly to a distinctly closing theme. All in all it retains some of that Mannheim quality that I pointed out having heard in the E Major sonata, HXVI:13.
Second Movement: Menuet
The word 'quirky' is overused in respect to Haydn but it certainly seems like the best description of this movement. The melody is delightfully syncopated, and an irregular phrase length in the second section adds to the sense of off-kilter-ness. The Trio is in the parallel minor (typical of an early sonata) and positively revels in irregular phrase lengths.
Third Movement: Finale Presto
A. Peter Brown considers this to be the most accomplished of the early Presto finales; it's a good call. Oddly fragmented, irregular phrases give way to Scarlatti-ish keyboard ostinato writing. The closing theme is in particular typical of later Haydn, ending as it does in mid-air. Also of note is the polymodal nature of the movement (typical of a much later Haydn practice) in which the parallel minor and major keys mix comfortably within the harmonic texture.