FORGET ABOUT SERIAL NUMBERS ON MOST BIKES FROM THAT ERA! 
In the late 70's the US CPSC (Consumer Protection Safety Commission) mandated that all bikes sold in the US had to have traceable serial numbers... It never happened!
There were a few bike manufactures and importers who had traceable serial numbers like Trek, some Raleighs, many of the Japanese made bikes and so on but companies like Gitane, Peugeot, Motobecane, Bertin and most of the other large French producers stamped random numbers on their dropouts and BBs! Most of those companies have been dirt side down for over 25 years!
END OF RANT!
The headset is a Shimano, maybe an early 80's 600?? The hubs are Campy, the cranks. brakes and FD are Shimano. You can't really judge much from a mismatched rear derailleur because they frequently got sucked into the spokes and replaced with whatever... like the Suntour Superbe.
This kind of component mix (all goood stuff) was common place of bikes built up from a bare frame.
OK so what do we have? If the seat tube lug is round, the 26.8mm seatpost narrows it down... Light gage French tubing or heavy gage imperial sized tubes like Columbus SP (or a lesser Columbus grade) and so on.
A light gage 28mm French diameter seat tube with a 0.5mm or 0.6mm wall thickness would take a 26.8mm seatpost
Heavy gage 1 1/8" imperial tubing like Columbus SP or Reynolds 1.0mm x 0.7mm tubing is 28.58mm diameter with a 0.7mm wall thickness and would normally take a 27mm seatpost.
It the seat lug was out of round someone could have used a 26.8mm seatpost.
Straight gage 1 1/8" seat tubes could have a 0.7mm, 0.8mm or 0.9mm wall thickness.
The easiest way to determine whether the tubing is metric or inch size is to measure the top tube - 25.4mm x 26mm. Add ~.2mm for paint. Measure in several places on the tube - side to side and top to bottom in case the tube is out of round.
The frame very well could have been made by a small frame builder. The lugs are Bocama Competition Ref 83 so with the Shimano components dates it in the early 80's.
Lug photos by Elton Pope-Lance - Harris Bike Shop
If the tubing is metric its a
"bātard"... If not then maybe it's a bastard!
Hope this helps a little.
verktyg
Crusty Chas.