Hopefully I remember this correctly,
Butt No.1 Mk1 introduced with STEN Mk1 Pilot and early production models.
Butt No.2 Mk1 introduced with STEN Mk1 Later models. Made by U&U (Unique and Unity)
Butt No.2 Mk2 introduced with STEN Mk2 Early examples made by U&U using the cast abutment plate of the No.1 Mk2
Butt No.3 Mk1 introduced with STEN MK3 made by Lines Brothers and then by Long Branch of Canada (Subtle differences)
I may have the Butt numbers slightly wrong because I can't remember where the Butt No.3 came into it.
Anyway, the T stock was the first of the common types followed by the loop butt (Rifle profile) not the other way around. The T Stock involved 5 parts to make, the abutment plate, the tube, the butt plate, the reinforcing plate and the machined stud on the abutment plate. All this had to be jigged and then welded. The loop stock has 2 pieces and one weld and is much more economical to produce. Now the STEN Mk2 and 3 were designed at the same time and the butts were also designed at the same time but the Mk2 was introduced before the Mk3. You only need look at the lineage of the Mk3 to see it is a simplification of the Mk1 not the Mk2