The superscripted values indicate the availability of 3rd party focusing screens, sometimes even available for models with a so-called fixed screen.
B = Brightscreens in up to 6 basic styles combinable with 6 grid styles (a.k.a. Crop Lines®)
F = Fotodiox manual focus screen with split-image
H = Haoda manual focus screen with µ-prism or split-image
K = Katz Eye™
Zeiss formerly also linked to Beattie Intenscreens (link dead, use Reflexite instead), but you might want to read this article first. Well, things might change over time, but it seems like Reflexite doesn't offer screens for any Canon SLR at the moment anyway. An old B&H catalog (2005) lists Beattie screens for Canon EOS series 1, 3 and 5 only, found one more on eBay for EOS 6x0 and RT family (that said, no screens for DSLRs at all).
The above article refers to products produced prior to 2000 and I'm not even sure, for how long Reflexite actually has been the manufacturer for this brand. Just giving some random bits of info here.
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