I believe the terms "Allen's Township" and "Allen's Town Township" were used a bit interchangeably initially, while at the same time Northampton was also used to refer to the town itself (probably not prior to the mid 1760s). Of course William Allen commissioned or drew up the well-known map of 1762 and there was a road petition mentioning 'Northampton' town of the same year - there is ample mention of this in the old county history. HSP has a map somewhere (I can't find the specific file reference at the moment) that is dated 1760 and contains a notation of "Allen's town" and the 1758 Horse and Wagon Census also notes "Allen's town." I don't believe these were abbreviations for Allen's township although I may be wrong and perhaps they are. I don't remember off the top of my head how it may be noted in the little publication the WPA put out in the 1930s of the 1761 county-wide tax lists but the Easton library should still have a copy of that.
The PA gazette has a notation from July 17, 1755 that mentions Allen's Town township: "Whereas there was a note of hand given the 11th day of November, 1754, by Joseph Brown, Blacksmith, of Allens Town township, Northampton County...". This seems a pretty clear reference to a location at least regionally referenced as Allen's Town, even if it is within a marginally larger locality of a township? That's my interpretation of it, anyway. Otherwise, why the redundancy?
I'm not clear on how to interpret the letter you posted - it reads "Allenstown" with no visible break. I can see it referring to Allen/Allens township or it could refer to a town location within the broader township, perhaps? In light of the PA Gazette reference I wonder if the two terms were being used interchangeably?
When I was rapidly posting stuff to my site I know I came across references to the actual location of what would become the town being inhabited by a small cluster of houses ca. 1739/1740 and growing from there. Unfortunately I don't know exactly where I found those references and I only briefly noted it on my site. It certainly was not being called Allen's Town at that point in time. This is one of the things I find so interesting about this map, however, because it clearly displays a symbol which appears representative of a cluster or settlement (judging by the use of the same symbol all over the map) and it seems clearly marked "Maguntsche" immediately above Emaus, although I can't quite decipher the black ink marked word or abbreviation between the two. This is the first time I have seen any representation of Macungie as a village or town around the location for later Allentown - I always viewed the Macungie area being further west, where it is also marked "Maguntsche" vertically on the map.