These things are always fun to speculate on.
I believe this unit is a 1988(ish).
1) The older Ingersoll 6018 units used an in-line canister hydraulic filter. The in-line canister is located on the left side -toward the front. If you look carefully, I think you will see the filter housing. BTW: In 1991, Ingersoll switched to a screw-on filter.
2) If this unit has the original engine, then I believe it is an Onan powered unit, not a Kohler. To my knowledge, when the Kohlers started showing up (1998?), they were 20 horse units and the model number was changed to 6020.
3) Starting in around 1995, the buckets on loader/LBH had L shaped brackets (kinda like "handles") on the back of the bucket at each end. I understand they visually aided the operator in determining bucket angle. This unit does not have the brackets.
Here is the part I can't resolve. This unit does not have a ROPS. The front mounting point for the ROPS is the side towers that support the lift arm pins. There is a hole in the tower where 3 hydraulic hoses (for the lift and bucket cylinders) exit the tower. The ROPS bolts on above the hole. This unit has telltale signs of bolt holes. This could mean one of three things:
a) The holes I think I see are optical illusions
b) The unit left the factory with a ROPS and it was subsequently removed
c) The unit is a later model because, starting in about 1994/5, all units leaving the factory had the holes predrilled, even if the unit did not ship with a ROPS.
Option (c) is the only inconsistency, so I vote is for a well-cared for 1988ish (our beloved serial number guide shows 6018 production beginning in 1989).
Somebody go get that thing!