This is D42, the Artillery Tractor Demonstration Model at Chertsey in 1929
This is the prototype Vickers 2 ton tractor, taking part in the February 930 trials at Wool, here towing a field gun and limber
A single 3 ton tractor prototype was delivered in early June 1930 but discarded as having no benefit over the 2 ton in 1932
The Tractor Truck mated a 30 cwt lorry body with a fully tracked chassis based on the 2 ton tractor and which came in a number of variants. Three vehicles were purchased by the British Army for trials but were rejected for service
Here are a number of versions:
Tractor Truck 1936 China
Tractor Truck Standard 1935
Tractor Truck tipping body 1935, also Metropolitan Water Board 1936
Tractor Truck Turntable and Trailer 1935, Turning Circle 26ft wit Load. Overall length with Corner extended 26'6"
Tractor Truck Aero Starter 1935
Tractor Truck Pipe Carrier 1935
The suspension (drive wheel and four road wheels) isn't the same as the 2 ton tractor, and therefore not Dragon Mk 1, 1a, II, or Tractor Truck, or Utility Tractor, all of which in any case wouldn't fit the given date of 1931. It's also different to D42, the artillery tractor prototype.
Which really leaves the vehicle in the last photo on my previous post (artillery tractor 1931 high sides) the only viable suggestion so far, to fit with the date, body style and suspension.
The later version of the 2 ton tractor used on the tractor truck does look similar to the Portuguese vehicle but dates from 1935/6 which postdates a 1931 vehicle.
It would be helpful to have a side on photograph of the vehicle in the Portuguese Army Museum to help further with identification. Obviously it would be simplest if the Museum told us what it is.