First of all I happy to hear you got an answer from Mr Tjaarda. Any pics of the prototype that we haven't seen before? if so, feel free to share!
Should you buy one? of course. I own a Deauville but my dad had a Longchamp for 5 years as a daily driver. Fuel consumption is 5 km for 1 liter with normal driving. Generally it is reliable. The electrical system is Italian 70's style but most of that can be replaced with modern and reliable systems.
Rust is always an issue especially with North European cars. That is the biggest worry. On the other hand, by most that are left over should have had a complete or partial restoration. Then you need know how well it was done.
Have you found a Longchamp that you want to buy?
I know there is one for sale in Germany. It's a brown one from, propably, 1976.
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