For more (much more) general information, I recommend the Nikon site and the Olympus site.
The above photos show a very small part of the Nikon filter offerings that have been available over the years.
Some of the filter types are: polarizing, polarization waveplates, neutral density, neutral color balancing, interference filters, diffusers, heat absorbers, and fluorescent dicroic cubes.
The inverted Diaphot uses 45 mm filters up top for diascopic illumination. There are two variations, one with captive filters in sliders, and the other with removable filters.
It also has a place for a permanent heat absorbing filter in the collector lens housing, which is a very good idea to install.
With the removable filter system, it can be used even if the plastic clips are missing, just using your fingers on the edge of the filter itself to handle them.
While I haven't seen every single variation on the Labophot/Optiphot 1/2 Series, I think that they all have, in the built-in diascopic illumination system, a permanent (a.k.a. not simple to remove during standard usage) diffuser or two is placed in front of the collector lens.
This frees up some real estate which would otherwise be used for a diffuser, but means that you cannot view and adjust for proper Koehler Illumination. The Optiphot with a 50 W diascopic lamp may be the only one of the line that has adjustments for the lamp, so there may be some justification to not having the diffuser be removable.
The Labophot and Labophot 2 both use 20 W bulbs in integrated lamp houses. For whatever reason, the Labophot 2 lamp house is prone to fail, so very few of those instruments exist with an intact lamp house. The original Labophot lamp house seems less injury prone.
The Optiphot uses a 50 W halogen bulb, in a lamp house very similar to the Diaphot, while the Optiphot 2 uses a whopping 100 W halogen bulb, in a pre-centered lamp house.
I do not have an Optiphot with diascopic illumination, but I'm pretty sure that this filter cassette attaches at the back of the instrument to the lamp house. The first photo from eBay shows the lamp house connection, and the second the cassette thay may attach there. I have seen a photo showing the Diaphot style removable filter clips installed here.
The Optiphot 2, however, has a very large set of slots in front of the lamp house. If you populate only one side of these strange looking filter holders, then you can flip it over to the empty side when you don't want that filter in. The following photo is from an eBay auction.
The Optiphot 2 and both Labophot models have permanently centered lamps, so can get away with a permanently installed diffuser, but with the Biological Optiphot, the lamp is manually centered, so the diffuser is manually inserted. The filters and filter holders are identical to that used in a Diaphot, 45 mm with half-moon clips.
There is also a cassette that attaches on the field lens housing up front. Orienting it as shown gives the most condenser stage travel. It has ND2, ND4, and ND16 filters in the sliders, allowing for some fine-tuning without having to go to the back of the instrument all the time.
I have never seen an official lamp and beam splitter cube setup for the Diaphot, but I suspect they exist. It is likely a xenon arc lamp and a 50% beamsplitter in the cube slot under the objective turret, but other light sources could conceivably be used.
I have seen at least three variations on traditional episcopic brightfield / darkfield illuminators (vertical illuminators) for the Labophot and Optiphot series. One uses an arc lamp, likely xenon, with appropriate filters included, another uses a halogen lamp house (usually 50 W, but can be 100 W), and a third has a port for a fiber optic illuminator instead.
The filters here are of the same types, diffuser, ND, NCB, GIF, etc, as usual. In the case of the "traditional" halogen setup, the filters are 25 mm in diameter, housed in a plastic case that is 42 mm wide. They slide into slots just in front of the lamp house.
Conditioning the source may not be the only filters in an episcopic illuminator. If reflected light observation with polarized light is desired, most units have provision to have fixed or rotating polarizers both before and after the beamsplitter. The one above the beamsplitter is called the analyzer and is often fixed in angle. The one in the horizontal path, just before the beamsplitter, is often rotated, so a special filter slider is desired here.
The special cubes used for epi-fluorescence imaging are composed of three parts: the excitation filter (which blocks part of the incoming light spectrum), the dichroic beamsplitter, or mirror, which separates spectral regions between transmission and reflection at 90 degrees, and a barrier filter, which blocks unwanted parts of the spectrum from being sent to the observation head.
Except for one special case, the Labophot, Optiphot and Diaphot series all use the same filter cubes. The exception is a Labophot/Optiphot equipped with a Quadfluor cube selector, which uses slightly different cubes.