As already mentioned by the above members, it might be hiding from you. A hint is: The slot will be located "across from" the point where the spring hooks onto the idler arm. It will be found somewhere near to where the idler arm is situated after the belts are in their respective pulleys and you draw the idler arm tight by hand. If you can do that, the slot will be easier to "discover". Once you locate the slot, it might be easier to take a belt out of a pulley, then engage the spring ends into idler arm and the slot, and then put the belt back into the pulley. You might have to "experiment a bit" to find what works best for you and your tractor

How to Optimize the Supply Air CFM Volume to & from the Rooms for balancing airflow to all rooms & upstairs rooms; in cooling mode, the Return-Air should also all come from upstairs. You want to greatly reduce the airflow ‘resistance’ on the return-side to produce more static pressure and CFM airflow on the supply-side at the diffusers. Any airflow needlessly restricted on the Return-Air side reduces the air availability to the blower wheel blades, which ‘reduces’ the blower wheel performance and the airflow through the evaporator coil and to & from all the rooms. You always want the highest static pressure on the supply-side at the diffusers to the rooms, while at the same time, as much as possible, reducing as much ‘resistance’ or pressure drops in the rest of the supply-side, which also includes low pressure drop fittings in all of the supply side ductwork to optimize pressure at the supply-side diffusers. You should also always have branch duct dampers installed so you can adjust the airflow balance, for example to 2nd floor areas, where that is a problem. Reducing all the return air ‘resistance’ possible on the return-side making any return ductwork as large as optimally possible, plus install large & open return-air grilles & filter grille areas as large as possible to reduce all return-side resistance and pressure drops possible, which will optimize supply-air CFM both to and from the rooms. If U use 1" deep filters I’d use Purolator model P312 filters; can't see through them & have low resistance & good filtering qualities; can order them over the Internet. Extra large Return-Air filter rack areas are always needed, even RA filter cabinets with 2 large slanted filters can be used. Very low airflow with some conditions, could lead to the freezing over of the evaporator coils resulting in possible severe compressor damages… If indoor airflow drops off, shut the outdoor condenser/compressor off immediately…! Also, always check the temp-split of the air off the outdoor condenser compared to the outdoor temp, the 'Expanded Cooling Data' will show you what the temp-split should be at existing conditions. CFM x WB Delta difference on enthalpy chart x 4.5 @sea level =Total operating Btu/hr Capacity...

Too bad. Sounds like a lazy, cost cutting design done without regard to the consequences. This would be a good thing to change and probably wouldn't be too expensive. Replace it with two or more smaller returns, one in a different location, with lower and more accessible wall placement. Or, as suggested, relocate where the filtration happens to somewhere more easily accessed. My vacation place has the HVAC equipment in the attic. The ceilings are 10 feet but the return grill has a 24 inch vertical length and is located on an upstairs wall. The two latches are at the bottom and are a bit less than 8 feet up. I can stand on a chair and get it easily, maybe something like that could be done for you.

Honestly, I would ignore it. :-) I'm not trying to be snarky. It's just that it needs to go somewhere, and a hallway is a good of a spot as any. It looks to be the same color as your ceiling. That means it blends in reasonably well. If you try to gussy it up in any way, it's just going to stand out even more and no one wants that. If that particular spot is just wildly untenable to you, it can be moved. How much that may cost will vary a lot, and will also depend on where else it may be able to go and still work as it needs to. I have several of them, scattered throughout my house. I literally never even see them, meaning I have to occasionally remind myself to dust them - they're that invisible to me. Curious if anyone else has any other ideas, beyond ignoring it.