Don't confuse Watson's Bactil model with its smaller brother the Watson Service model - the Bactil is a much bigger (and heavier) model, the largest that Watson produced in this era. The earlier Bactil models like this one were produced with a black finish, whereas later Bactil 60 models (introduced in around 1960) were in an off-white finish, so this example is a classic Bactil model in its period black livery.
eBay