Interesting security thread

This has a solid security thread, meaning it is one line as viewed under backlight:

What is unusual (at least for me) is that the thread is not the same density throughout. 

What other notes have this feature? Is it rare?

Today I X-rayed this note, to find out whether the density of the thread changes as the backlight scan suggests. 

Indeed it does:

Interesting. It might suggest ‘secret’ security features in banknotes which are not widely advertised.

I agree.

I X-rayed more than 50 notes and although I haven't analysed them thoroughly I can tell you that a lot of security threads that look solid under backlight have variable attenuation radiographically. The attenuation differs in the long axis mainly but I've seen it vary across the width also.