Release date of the 520STFM?

Hi All,

I'm trying to figure out exactly which year we bought our brand new, just released 520STFM. I know it had TOS 1.0 as it played Arkanoid no problems and also it had single sided drive. I'm fairly sure it was 1986? It came from Silica Shop & included NeoChrome & 1st Word, that was it. I know the huge game bundles came later right?
Also anyone know when the double sided drive became standard?
Finally when was TOS 1.04 installed as standard on the FM, just trying to date the machine I have now.

Best regards,
Chris