Lots of good information above.
Originally posted by Pdean44 The lens in the camera is smc-pentax-m 1:2 50 mm 4759859
That's what we call an M50/2, a very common (and somewhat under-rated) kit lens of the 1970s. SMC-PENTAX-M refers to the lens series (M-type). Other numbers indicate the maximum aperture (1:2 or f/2), the focal length (50mm), and the serial number (which might mean it was made in 1975). The lens will work on any modern Pentax dSLR.
Ah, the filters. If you plan to actually shoot film with your new cameras, you will want a UV filter; if shooting B&W film, you'll want a set of B&W filters (yellow, orange, red, green). A polarizer would be handy. Your diffuser and center filters will be useful for portraits.
The split filter could be of several types:
* Split-focus, for apparently extending DOF; half the filter contains a close-up element, the other half is clear glass or empty
* Split-ND (neutral density), for shooting scenes that are half-bright, half-dark (like landscapes under bright skies); half the filter is shaded, half is clear
* Split-contrast, to apply a B&W filter to half the image, like to make clouds stand out without changing a face's qualities; half the filter is colored, half is clear
Split filters and polarizers are useful with digital; the rest are for film only. If shooting color film, color-correction filters may be necessary. I hate those!
We typically don't want to spend a lot of money on multiple sets of different-size filters. Pentax made many many lenses with 49mm front threads just so that only one set of filters would be needed for all lenses. If we have a few lenses with 52-55-58mm threads and more with 49mm threads, we might buy one set of 58mm filters, then use step-up rings to fit them to the narrower lenses. Good filters, especially polarizers, get EXPENSIVE. Ouch. One of each is enough, thanks!