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I've seen several examples recently at E-bay of databacks which are purportedly interchangeable between these two models. My primary resource is:
Backs for the Pentax K-Mount Bodies
Dmitrov indicates that they are not the same but has no explanation regarding the difference. An overpriced item came up today:
Asahi Pentax Dial Data ME\MX with Box and Manual | eBay
The item is clearly marked ME but the Manual may have been printed for two closely related models.
What is the difference?