I believe they are the same. I had my doubts: i have the Positivo Alfa, which is exactly the 4fff's N618, as the NVSBL L337, the Promedia E-Reader and the Bq Avant: same external case, same 7 buttons, etc.
Then I found Frank's Wiki about the Oyo, and as you can see he lists the Avant and Promedia as the same devices:
http://www.fwma.de/pmwiki/pmwiki.php?n=Main.OYO#toc24
Since Sipix and Benq are controlled by AUO, it is probably a Benq / Qisda project, using Sipix screen and other AUO technologies. So, I believe all the models are indeed the same, with little differences as the positioning or number of the buttons and presence or not of 3G, according the retail seller's choice and desire.
I was wondering if I should update the Wiki, and if so, should I complement the Oyo's page or write a new one?