Enigma Settings Sheets

Hello Bletchley Park list!  I've been subscribed for a little while so I
hope you don't mind me leaping in...

I see in years past there has been some excellent in-depth discussion on
this list of the Enigma machine, its keys and code-breaking techniques.  Is
it still appropriate to discuss that here?  (If not I'd certainly
appreciate any pointers to appropriate mailing lists or forums.)

I've recently created an Enigma settings generator web page (
http://goo.gl/T80zL ) as I wanted a few things that Dirk Rijmenants'
Windows tool doesn't do (including support for the pluggable reflector,
D).  This is in conjunction with an Android Enigma simulator (
http://goo.gl/EwywN ).  I have read Philip Marks' excellent papers on UKWD,
but I have a few remaining questions:

- Was there any particular logic governing when UKWD settings changed?  The
authentic settings sheets seem to have used variously 7, 8, 9 or 10 day
periods.
- Was there any physical limitation on which pairs of letters could be
plugged together?  The authentic settings sheets avoid alphabetically
adjacent pairs (like 'AB') but not all physically adjacent pairs ('IK' is
actually physically adjacent, because there is no 'J', and that's used on
the Luftwaffe settings sheet No. 651).
- Why did they avoid alphabetically adjacent pairs on the plug board in the
first place? (I assume this was a key generation practice that was simply
carried on when the UKWD settings were added.)
- Were there any rules about which ring settings were permissible (such as
not using the same one two days running)?
- Are there any authentic settings sheets available for earlier indicator
procedures (with specified Grundstellung and/or fewer than 10 plug board
pairs)?

I have a couple of other random Enigma questions as well, so I may as well
ask while I'm here :-)
- What's the difference between an Enigma M2 and M3?
- Deavours and Kruh 1985 (and hence Wikipedia) states that Enigma models
prior to I used a different rotor construction, where the turn-over notches
are fixed to the rotor core rather than to the settable ring.  Is this
true? (I haven't seen any other reference) and if so, which models?

Thanks!

- Craig Heath.