Hello. As you know, Russian cast-iron trading weights of a spherical shape entered the practice of weighing in the 19th century. At that time, the law obliged merchants to have weights of up to 2 pounds. However, we are aware of specimens weighing up to 5 pounds. A weight heavier than 2 pounds is extremely rare, and their scope is not entirely clear. Perhaps they were used for weighing in warehouses, in wholesale trade, for weighing heavy loads, such as, for example, loaded wagons. There are also references to the non-standard use of 4-pound weight weights. For example, two such weights with long chains were in the Avramiev-Epiphany Monastery (Rostov the Great), which were entrusted to the ascetics "for greater labor of the flesh." In the “Guide to the Urals” from 1899, another story is found, according to which a weight of 4 pounds with a chain was used to punish serfs, who, in case of disobedience, were required to literally move with a weight around their neck. According to another version, the watchmen were chained to the same gear so that they would not leave their post.
The letter markings on your kettlebell are most directly related to the place of production. The abbreviation "PZ" means the Peskovsky factory, and the letter "M" is probably the monogram of the owner of the enterprise, merchant A.I. Malikov, whose property the plant was from 1842 to 1858 (this is about the time of casting your weight). By the way, similar designations are also present on the weight known to us weighing 5 pounds. Another noteworthy point is the designation of the face value. In addition to the Arabic numeral "4" on the gear there is a designation "IIII", somewhat reminiscent of the Roman numeral. The tradition of double denoting the face value on Russian trade weights was quite widespread until the twentieth century, since it was believed that even illiterate peasants were easier to remember Roman numerals (remember sticks for counting). Probably, for the same reasons, the manufacturer decided to replace the standard Roman numeral IV with four Roman units.