Google let me post it this time, but shows my actual name, rather than
one of my e-mail addresses. Sometimes it does one and sometimes the
other; I don't know why.

After I posted it I noticed an error in the long paragraph; it should
read:

"Inside the finger holes there were arrowheads
pointing out to the numbers on the ring, rather than the dots which
the 500 used. This was known as the 'C' plate. The 'L' and 'F'
plates were also made for the 21 dial to enable it to be used as a
replacement for the 10 and 12 dials in older instruments. The 'F'
plate was also used, with a blank outer ring, after all-figure numbers
were introduced some time in the '60s. The 746, a slightly updated
version of the 706 was fitted with this arrangement."

When the 'L' plate was used with late or refurbished 706s, and with
all 746s, a clear plastic finger wheel was fitted; this was also used
with the 54a dial.

I also typed 'finger' somewhere that I should have typed 'ringer', but
that one should be obvious.