Mine has a spring hinged latch to keep the visor down. Sir Wolf I think they are talking about the hinge to attach the visor to the helmet.
I ordered the helmet from an armorer from Ukraine I met on AA who worked with Jolly Knight to finish some of his work load. Dmitry Vasilchenko but he is best reached through the AA and his username is: Dietrich Vasil
Gareyth in theory if the bascinet is round enough or shaped with the Great Helm (one order) it should be totally doable! That system was very common in the early to mid 14th century. Do be advised that the Great Helm is going to look huge!
Dmitry I believe charges 300 for the visored bascinet and I paid 40 extra for the bargrill attachment. So for your system with a great helm you will probably be in the 500-600 range (I think including shipping) but only he can tell you really.
His english is sketchy but he understood what I wanted and delivered! Well...once I get it I guess he will have delivered