And the good news is... (for me, anyway) according to both Ron Wheeler and Timothy Harris, who kindly proffered their opinions upon close inspection of this vase, it's all as it should be! As far as I understand, the reason the top rim is polished is because this particular square shape was blown into a mould, and when the vase is removed from the mould in its cooled down state it leaves a rough top and bottom. As the vase is no longer hot at this stage the top cannot be re-heated to smooth it, so it's 'cold worked' (polished) to finish, as is the bottom - which is usual on many blown vases anyway. The size of it (and the signature) are all correct. Mr Wheeler has never seen one numbered higher than 275 (of 500). These vases were intended for export to the USA but the intended 500 were not ultimately required.
I must also say that I originally purchased this vase on ebay from sarahatherton64 who was - if there turned out to be anything wrong with the vase at all - quite happy to refund me. She was only interested in me being 100% satisfied with my purchase and never made me feel uncomfortable about questioning the possibility of any issues with this vase after I'd purchased it from her. So I can certainly recommend her as a ebay seller.
My grateful thanks to Ron Wheeler, Timothy Harris and Sarah Atherton!