Wanted: (Campag) seatpost bolt, 21mm-ish and some advice

Over-torqued my seatpost bolt when adjusting my saddle height mid-ride after swapping out cranks, and had to ride 10 miles back with the seatpost all the way down. A few questions:

1. I’m hoping I wouldn’t have damaged the frame by riding a saddle without a seatpost bolt? It’s 753 tubing with an alu seatpost if that helps.
   
2. Is there any reason why I need to replace this with a campag bolt? They are frightfully expensive (£30 for a bolt on eBay seems like daylight robbery) so I’m wondering if there are cheaper alternatives for steel frames which might work in its place (I see unbranded ones on eBay for a fiver)? Would be fairly keen on a non-QR one to deter opportunistic theft; I liked the fact that the Campag bolt needed two Allen keys.
   
3. Anyone got one for sale? Think my seatpost spacing was about 21mm long based on the snapped pieces, and think the inner diameter of the seatpost hole is just under 8mm (my 8mm Allen key does not fit in the space, but just about).
   
   

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