I don't know the dates, but Celestion "recently" took their production back from China and resumed production in England.
That being said, the Chinese made Greenback reissues and the English made Greenback reissues are exactly the SAME.
Celestion shipped the manufacturing equipment to China, and Chinese workers made those speakers on the same tooling that had previously been in England, and all the parts were the same, all the assembly procedures were the same, the only difference was where they were assembled and who was doing the assembly. Now all that tooling is back in England and English hands do the work.
If I'm looking for a reissue Greenback, I don't care if it was made in China or England. They're the SAME. I have both. I've compared them. They're the SAME. Right down to their sound.
I don't know which models have bounced back and forth between China and the UK, other than the GB reisssues.
It's my understanding that if MIC, the magnet sticker will have a number "50" as the manufacturer code. I would assume the OP's is a UK build based on the photo they provided that displays "IPSWICH ENGLAND" on the magnet sticker.
I am planning on installing these Vintage 30's as the bottom two speakers in my 4x12, with two Blackbacks filling the top two positions. Should sound good I think...