The answer may be in the description “trusty old”. The ID has a wear indicator cast into the friction bobbin. It is located at the top of the bobbin on the side of the bobbin that the swinging side plate is on. When in usable condition the wear indicator is visible as a slightly raised ridge about a half-inch long. If the wear indicator is not visible the bobbin is worn out and the ID needs to be taken out of service.
Smart answer courtesy of Pat Furr