There's nothing rare or special about this freewheel except it's age. These types of freewheels were very common in their day, and were later replaced with 4s and 5s freewheels that stepped down smaller than the hub thread, and overhung the axle making smaller sprockets possible.

Like classic freewheels it unscrews from the left by means a of a freewheel remover that fits the inner body, and is built using a left hand threaded bearing cone jammed tight to a shoulder and loosened with a punch or 2-pin spanner. It's bearing play is adjusted with thin shims under the cone, which are removed one at a time as the parts wear.

These can be disassembled and rebuilt with little effort off the hub, but are almost impossible to work on with the wheel attached. If the OP wants to service it he should slightly loosen the cone, then stand the wheel in a vise with the freewheel level on top, remove the cone very carefully to expose the bearings, but making absolutely certain not to lift the outer body, lest the back bearings (1/8" or smaller) start dropping out.

BTW- with the axle out, you can improvise a piece of steel to span the two notches and remove the freewheel. In similar situations I've made removers from large bolts, filing off the corners except for two tabs to fit the F/W then with the bolt held in a vise removed the freewheel by turning the wheel.

I may also have the proper remover in my collection (to lend, not sell). If the OP will email me the ID and diameter across the notches, plus the width of the notches, I'll check and confirm, then we can work out details.