1842-1879 most likely refers to co-ownership of the Nichols & Plinke factory.He was a shareholder, not a subcontractor.
Фабрику фирмы на Лиговской ул., 38 с 1842 по 1879 гг. содержал купец 1-ой гильдии Роман (Роберт) Яковлевич Кохун.
The factory of the company at Ligovskaya St., 38 from 1842 to 1879. was co-owned by the merchant of the 1st guild Roman (Robert) Yakovlevich Kohun. Qrt.S assumption in regard of 1842-1883 period makes sense because we don`t know what happened with his activity after 1879.
This is full text from Skurlov (the first year of his activity was 1865).
КОХУН Роберт (Роман) Яковлевич. Содержатель Т. Д. "Никольс и Плинке". 1865 г.: 68 л., кавалер, в купцах с 1842 г., куп. 1-й гильд., прож. в доме Васильчикова по Невскому пр., 16. Т. Д. под фирмой: "Никольс и Плинке". 1874: Серебро: 72 п. 13 ф. 61 з. Также прож.: Лиговка, 38, фабрика, разные бронзовые предметы, 3 пары двигателя, объем производства 300 т. р., 138 раб. Постникова—Лосева: клейма №№ 1395, 1396, 1397.Содержатель Т. Д. "Никольс и Плинке". (shareholder of Nichols & Plinke joint- stock company).So, he was mentioned before the factory, i.e. in 1865, and as Qrt.S observed well after the factory.