Sorry to bump this already, but I'm eager to go ride and I hope someone might know the size. The current clamp is QR and just doesn't keep my seat post from rotating and creeping down.
Yep, 27.0. Sheldon Brown states it's a 27.2, but I've heard it was 27.0 so that's what I've got in. I generally have the issue with creeping seatposts on all bikes due to my weight, unless I use a bolt clamp.
I'm surprised that there has been no response from an owner (past or present) of a 1995 Explosif to confirm this.
With a view to help with your question, I will say this:
BrushedAluminium is quite correct that the Explosif from 1995 is Steel, and requires a 28.6 Front Mech. I haven't owned a 1995 Explosif, but my 1997 Explosif used the same Columbus Max tubing - albeit with somewhat different enhancements - and it required a 30.0mm Clamp, as Columbus Max tubing is slightly externally butted at the Seat Post Clamp area.
Here is that very frame with a Hope Clamp fitted, which is etched with 30.0. The bolt area is sitting with parallel faces, so I would say it is the correct size for 1997 Columbus Max tubing.