What year did Yamaha Pianos switch from slotted (-) screws to positive (+) screws?

Yes folks, that is the hot topic for today. Which year did Yamaha start using positive/phillips (+) screws on their Japanese upright pianos instead of the older style slotted (-) screws?

So far all I have to go off is a Facebook comment claiming that it happened in approx. 1985

Well here is my evidence

• This 1980 U3 has NEGATIVE screws
• This 1983 U1 has NEGATIVE screws
• This 1985 UX1 has POSITIVE screws
• This 1989 U30BL has POSITIVE screws
• This 1989 UX10BL has POSITIVE screws

The evidence so far looks quite convincing that 1985 was indeed the year. More research needed to check some 1984 models when I find some

Cheers
Mark