The early model Mk23 LAM (PEQ-6 / Insight Model 400), did not have an accessory cover for diffuser or pattern generation. The original intent was for a IR filter to be affixed to the white light illuminator for close-in work and use within enclosed or indoor spaces. The halogen gas bulb which is standard for the Mk23 / Mark 23 LAM outputs a prodigious amount of infrared light in addition to visible white light. When filtered with 850 nm IR bandpass filter, it outputs just the right amount infrared illumination to be useful and not overpowering to NODs / NVGs within 25 yards within enclosed spaces.
It wasn't until the emergence of the PEQ-14 requirements nearly 10 years after the first Mk23 LAM that the IR diffuser and then the pattern generator covers became an integral part of the PEQ-6 design. The reason for this is that at about that time, the industry in light optics began moving, in earnest, from incandescent technologies to the much more robust, cooler operating, and more energy efficient LED lamp technologies. Knowing that LED lamps for white light were the emerging trend, and knowing that white light LED lamps output little to no infrared, the military requirements engineers added the requirement for diffusion lenses for the IR illuminator on the ILWLP. Concurrently, the use of high powered IR laser pointers as sparklers and signalling devices surged in the U.S. Military's combat operations, and thus the adaptation of the lens covers to hold holographic "pattern generator" lenses.
Once, on a lark (aka 'shits and grins'), I opened a PEQ-14 and relocated its internals for IR laser aiming and illumination to a LAM-1450 (VIS only model), after drilling out the escutcheon on the LAM-1450 to make way for the IR laser aimer and illuminator apertures. It turned out to be quite a chore to both disarticulate the PEQ-14 and retrofit the LAM-1450 as the electronics in both are all potted in epoxy ... about as tedious as complex as removing a fossil Tyrannosaurus Rex skeleton from Gobi Desert red rock with dental tools and a small brush. It took months and months of patient labor to free the transplant components and then to re-connect them. The result was remarkable, though and when I got the converted LAM-1450 re-potted and fully assembled, it was impossible to tell the difference from a LAM-1400 without looking at the old label still on the unit.
A field-expedient diffuser for the IR illuminator is very easy to craft. Just use a hole-punch to knock-out round blanks from a section of semi-transparent Scotch tape. Stick the round blank over the IR illuminator lens and it will diffuse the output. If you need more diffusion, stick another round blank or several more on top until you tune the diffusion to where it works best for your application or need.
An interesting project for folks with optics engineering background is to build a CAD system for holographic image transfer to acetate film using a common laser printer. There's freeware software routines for this work, but tuning the offset for the holographics requires quite a bit of skill and knowledge of how to manage light as vectors. For fun, I've developed custom pattern generators for the IZLID 1000, MPLI, etc. that project images of agency logos, corps insignia, and the occasional pin-up girl onto distant surfaces (e.g. tree-line, mountainside, etc.). The IZLID and MPLI both use Butler Creek flip caps on their objectives, and therefore it is a simple matter to cut an opening into the lid / cover and affix the acetate film to the inside rim.