Tire with no ETRTO/ISO size
Older tires may not have the ETRTO/ISO size markings. On your tires the '26x2.1' marking denotes rim diameter and tire width.


'26 inch' mountain bike tires have 559mm rim diameter.
2.1 inch is the nominal width. 


With an 21-22mm inner rim width you could run a 1.5 inch / 38mm tire, but probably not much narrower.