The three main components laid out. Removing four screws allows the cover-plate to be taken off, undoing the two nuts under the machine allows the baseplate to be taken off.
The main frame of the little adding machine is riveted together, so cannot easily be taken apart. Nevertheless, with the baseplate and cover off, much of the dust and dirt it has accumulated since January 1939 can be brushed away. The mechanism internally is dominated by the large wheels and segments used to enter a number, adding the number of digits to the totals-register as they are being pulled down.
Another thing is the stylus - that is often missing with these calculators. Browsing the internet gave information on what the original stylus probably looked like. The post-war calculators seem to have had a green, shaped, double-ended stylus - the 1930s specimens probably had a simpler, black stylus. From an old pencil and a suitable nail a credible reproduction stylus was crafted.
Cleaned and now with a stylus, the Resulta BS-7 calculator works fine. It is a basic adding machine, with subtraction too. It does have a feature for quick-repeat adding to do multiplication, but I would not characterise this as a four-species calculator.
In the picture above, 12.75 was entered and the input cleared. Then 7.95 was added; the 7.95 showing in the lower input check-register. The sum is shown in the top totals-register. The crank at the top-right clears the totals register again. The lever at the left shifts the machine into subtraction-mode.
A fun little (and surprisingly heavy) addition to the small set of vintage calculating tools :-)