This is the instruction sheet from a set of the three achromatic, 2-element
close up lenses that were originally made for the SRT series. They were made in 49, 52mm and 55mm sizes. The 55mm ones are much more common and are sometimes available on e-bay. (they are no longer produced) The ones I have are coated and appear to be of very high quality.
You will notice that the "0", "1", "2" designation is not a diopter strength. The "2" lens is close to +4 diopter as far as I can tell. I will be doing some testing of these lenses for chromatic aberration and flatness of field with the D-7. These are not ordinary close up lenses, they are more than twice the thickness of a single element lens and are satisfyingly heavy.