Orange frame number dilemma…

[Commanderrb]

Loads of dating help needed lately!
I found a thread on orange frame number dating and just for interest was looking at mine which I’ve always though was around 89 or 91 (Ive two like this, the other is the Lemon colour scheme).
the frame on this one is A609/1205 and from the other thread I thought second digit would be like “1” for 1991 etc, so the 6 has thrown me.
I need to drum up the energy to go and dig out the Lemon and look at that.
any ideas What this frame number / the 6 is telling me?

 

[rwm1962]

They don't make sense to me until 91/92 when first 4 characters denote plant, year & month.

 

[Commanderrb]

These were two NOS frames Orange found at the back of their place a few years ago, they said they may have been pre production / intended for their people only so wonder if that explains. I'musre they reckoned they were both 89 (they were in the same box they said) but I've forgotted and keep getting 91 in my head. The Lemon is A89144429 which makes more sense - 89 maybe, and this one is printed accross the width of the B shell rather than sort of 'around the curve' of it.