It isn't the font you are looking for but maybe a good alternative:

http://www.houseind.com/fonts/neutraface

And duplicating your former request won't let you getting more answers ;)
That said, what's your sample origin? Could make the search less unsatisfactory...

@Krijn, close, but still a bit far away. Thx anyway.

@Ryuk: well it helps sometimes. When the message gets bumped to the bottom nobody sees it anymore. ;-)
I'll try to get some more background info on the usage, but I think the client also doesn't know much.

Strange that nobody can recognize it. But hey, I also can't.

I think it’s not a typeface but lettering…

Looks like a lettering style taught to painters/decorators. They used sample manuals.
You could try to get some help from the experts at the Zuiderzeemuseum in Enkhuizen. There is a crafts-and-arts live exhibit there, if I recall correctly.

It IS a handmade font. The client dug in his papers and found out a Belgium designer made the font based on an existing font.

Thanks anyway for you help!

May be you'll find your way reconsidering my suggestion of Giorgio Sans (with alternate /O from titling caps). A quick and ready-to-go solution.

The sample—which I also concur is hand-lettered—displays many similarities (plus one glaring difference with the /M) with Saul Hess's Twentieth Century Poster, a version of which is here...
http://www.myfonts.com/fonts/nicksfonts/renard-moderne-nf/