I'll second all of Andy Warren's comments about the horrors of inconsistent
PIC part numbers.

From past experience I can guess that what is probably happening is that
Engineering makes up new part numbers in a manner that is at least somewhat
consistent, and then Marketing screws around with them.  Someone please tell
the Marketing people to keep their hands off.  Engineers are the ones who have
to decide whether the application calls for a 16C70, 16C71, 16C71A, 16C710, or
16C711, and it's getting difficult.

And the data sheets really need a section explaining the difference between
the parts.  What's the difference between a 16C54 and a 16C54A?  You have to
spend hours comparing the data sheets side-by-side to find out.

It doesn't help that Tech Pubs decided to throw dissimilar parts into
humongous data sheets.  The 16C61 and 16C71 should have been in one data
sheet, as they are almost identical, and the 16C65, 16C73, and 16C74 in
another.  It's ludicrous a single data sheet tries to cover the 16C71 and
the 16C74, which are much different.  Not to mention that when this
"grand unification" occured some details from the old data sheets got lost.
I still keep my old 16C71 and 16C74 data sheets around because even though
they may be less up-to-date, they are a hell of a lot easier to use.

What the heck are the "FR" parts anyhow?  I read some of the product briefs
(perhaps not closely enough), and I couldn't figure it out.  Somehow I
would have expected a simple one-sentence explanation of this great new
"FR" feature to be prominently displayed.  No doubt it is another Marketing
breakthrough, like calling byte-erasable conventional EEPROM "Flash".  The
whole point of Flash is that it is supposed to be cheaper than conventional
EEPROM because a real Flash cell is only trivially bigger than an EPROM cell,
rather than the *much* larger conventional EEPROM cells.  Maybe someday
Microchip will license some real Flash technology; then they can get rid of
OTP entirely.

Cheers,
Eric