A lot (all?) Belgium Diamant frames were built by Martelly.
https://www.martelly.be/
He built frames for a lot of smaller brands in Belgium and the Netherlands, like Cornelo and some from the Dutch brand Union. I really enjoy their paintwork, Diamant and Martelly are often really nice looking. Very much racing bikes, not much for touring I assume.
I heard you can contact him and ask about an old frame you have and he does sometimes reply with some information.
In Belgium they are widely available second hand. Jus look here:
Martelly on 2dehands.be
And here:
Diamant on 2dehands.be