The drop in filter you see in that lens is part of the optical formula. There probably should always be some sort of filter in there. As it's labelled "Regular" I would guess it's a plain filter in there now, with no effect other than to maintain the optical formula. In addition to a holder that's threaded to use screw-in filters, there are also drop-in gel filter holders and polarizing filters that are permanently installed in a unique drop-in design with external controls to be able to adjust the filter's effect.
The non-stabilized super telephotos all use 48mm drop-in filters like yours. All the later IS lenses use a larger, 52mm drop-in filter. The 48mm and 52mm types are not interchangeable. Note: There also are smaller 34mm Canon drop-in filters that were used in the FD or FL manual focus lenses. Plus there are now select Canon drop-in filters for use in an EF to RF adapter, unmarked for size. I don't know for sure, but would strongly suspect neither of these types are interchangeable with the 48mm size your lens uses.
It is not possible to use a screw-in C-Pol or any Variable ND filter in the 48mm and 52mm drop-ins. Only the special C-Pol and fixed strength ND can be used in them. The modern version for use with the EF to RF adapter has a Variable ND drop-in available, as well as its own C-Pol drop-in.
I remember reading somewhere that a guy had purchased a lens like mine but his had a polarizing filter in the slot and he was unsure as to whether or not it had to stay in the slot!
In the case of the C-Pol, if that is all they have it will have to remain in the slot. If it were removed and another drop-in were not put in its place, there would be a massive light leak at the rear of the lens ruining every image.
He would need to get either the 48mm gel or 48mm screw-in drop-in filter holder. While it's recommended some sort of filter always be fitted, those can probably be used without an actual filter installed without much of any harm. The holder itself will serve to close off the slot and prevent light leaks. But because of it is a special assembly, with the filter permanently installed and a dial on the outside to adjust the filter's effect, this isn't possible with the C-Pol drop-in.
48mm gel drop-in filter holder for sale on eBay.
48mm screw-in drop-in filter holder at B&H Photo (currently unavail., watch for used here or elsewhere).
48mm C-Pol drop-in filter at B&H Photo (currently unavailable, watch for used here & elsewhere)
48mm C-Pol drop-in filter at KEH Photo (presently out of stock)
As to the user manual...
I have only the later IS lenses and the manual for those will not help you. They share a single manual for the 300mm f/2.8, 400mm f/2.8, 500mm f/4 and 600mm f/4 (possibly the 800mm f/5.6, too, though that lens came later). Possibly it's the same with the non-IS lenses, that they share a single manual and all have similar controls?
Whether that's the case or not, I also didn't find the manual specifically for your lens online.
I did find:
Ken Rockwell's review of the EF 300mm f/2.8L USM lens.
MIR.com web page devoted to EF 200mm f/1.8L USM lens, with good illustration of similar controls (scroll down).
Canon USA Support: Using EF 400mm f/2.8L USM Focus Preset Function.
Scribd PDF of EF 500mm f/4.5L USM lens manual (preview only, account and download req'd).
YouTube video showing EF 500mm f/4.5L USM lens in use (on current R6 camera).
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2) some other cameras, various lenses & accessories
FLICKR & ZENFOLIO
