Dear Geraint,
Thank you very much for your very helpful reply. My thoughts were similar but you've expressed the issues far better than I could have. The 39 year gap is a long one and without further info coming to light, it may as you suggest, be down to a shinsa panel. It is, as you kindly say, a lovely naginata and it may be worth the polish/shinsa route. Before that however, the (lovely, period, full length) pole it is mounted on needs the raden and fittings on the tachiuke restored and that'll be the focus for now. I might post it in a new thread to enquire about potential restorers in the UK. Meanwhile, I will continue to enjoy the blade as it is...... :)
Regards,
Roy