To my knowledge no difference between "production
cel" and "original production cel" apart from the
person selling the latter is just trying to
provide extra assurance to you that it is a
legitimate production cel. If it says production
cel, then that means it should have actually been
used in the production of an animated work. 

A1End and B12 and labels like that refer to the
order a cel had in a particular sequence, along
with whether it was the starting cel, ending cel,
or a key cel. Designations like 001 or 112 can
denote a particular number in a reproduction line
of cels (aka limited editions or something like
that) but in the case of animaxis, it just denotes
the order in which they put the cel on their
website. Not that animaxis doesn't sell repros,
but if it says it's a production cel, then it is a
production cel.

You can see more Now and Then, Here and There cels
for sale on Yahoo Japan here:
http://search.auctions.yahoo.co.jp/search?sb=desc&desc=%a4%bd%a4%b3%a4%cb%a4%a4%a4%eb%cb%cd+%a5%bb%a5%eb%b2%e8&auccat=0&acc=jp&f=0x12&alocale=0jp&mode=1

These can be purchased through the aid of a deputy
agent such as Rinkya or ShoppingMallJapan.

I hope this was helpful information for you. ^_^