Originally posted by kaseki Because the filters mount at the rear, one would expect that one filter would have to be mounted at all times to keep the focal point where it is expected to be. Lack of a filter could also increase chromatic aberration at the film plane (or focal plane array), if the optics design is based on a filter being there. Is the design such that absence of a filter has only the effect of requiring a change in focus? Is the focus ring distance calibration intended for a filter being present, or not present? Note: back focus would be expected to change by approximately one-third of the filter glass thickness.
One clear glass filter was installed at the back of the mount when I received the lens. The inscription on this filter reads: "r-1.4x" (where the r appears to be an upside down L and the x appears as a superscript) AND "38x0.5"
Three additional filters are present in the cap of the leather lens storage tube. Their inscriptions may be observed in the attached photo in this order: green, amber, and clear.
I have no further information, and therefore cannot answer the two questions raised in your post.
Trusting that this information is relevant to your concerns.
Note: Sorry, the wrong photo is included, so I will post a second note with the correct photo. (Attached photo was taken with 645D and Pentax-D FA 90mm f 2.8.)