Have a look at this
Pentax 6x7 Lens Guide
Pentax identifies on their lenses the following markings for the latter two generations:
- SMC Pentax 6x7 55mm f4
- SMC Pentax 67 55mm f4
These are two different lenses both in generation and optical design in this case. The 6x7 and 67 designate the generation with 67 being the newer one. Most of the 67 lens generations have only cosmetic changes to the housing. That lens guide will tell you if there was a change in the optics. Note, too, the lens guide rates these lenses relative to
other Pentax lenses.
In the case of the 45mm and 300mm, no change was made in the optics between the 6x7 and 67 generation but you'll see a cosmetic change. The even newer M* 300mm f4 ED lens, however, is different and less confusing to know the difference by the name except the lens guide doesn't identify its name fully as it appears on the lens.
When shopping for lenses online, often the 6x7 and 67 nuance is not noted and generically refer to the lenses as 6x7 with only "late model" noted as to distinguish between the latter two generations. So you really need to be careful if it matters. And of course there shouldn't be much confusion with M* ED lenses.
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