Thank you for this post. This helped me solve my problem too.
I want to ask a further question:Regarding "the bolt head has a spigot / line up so it does not turn," on my 30 year old bike I noticed the area where the spigot goes in (on the seat tube) has worn out a bit because I never knew about the spigot on the bolt head until today. It still is ok and it does not turn but if this was totally worn out, are there other options to prevent the seat from collapsing into the tube? Can one put a bolt and nut to just tighten it (not good cosmetically) and use something else to keep the seat from moving?
Thank you.