Royama watch with Grilamid Base

[Juleswatch]

Hello from chilly Brazil!

I just bought a couple of watches and this watch came with them. While perhaps not worth much I'd like to try to find out something about it. Googling comes up with with the name Gerald Genta, and I see from internet searches he's made some nice designs (perhaps an understatement)! Also some cartoon designs but I can't find anything like this anywhere, so most likely not his!  Googling Royama I see that some famous people liked his watches and also the Royama watches.

Two questions;

1. Can anybody tell me anything about this watch and its background?

2. How to get the base off to let down the main spring which is fully wound. I gently tried prying it but nothing, I see there is a button but have no idea if it is functional for removing the back or anything else. Screwed back maybe? I really don't want to break the back or the watch with too much pressure and haven't looked up the properties of Grilamid!  

Probably not much of a watch but it peaked my curiosity and I hope somebody might be able to help?

The auctioneer sold it as a psychedelic acrylic 1960/70 Royama watch overwound!

All the best...Jules

[AndyGSi]

Are there no obvious notches or indents around the back?

Is that just a rubber plug?

Normally something like this would be to access the balance for adjustment
without removing the back, but it seems in a strange position for that.

[Juleswatch]

Nothing obvious I can see, I'll take a better look today. I thought so too, I thought maybe a button to remove the crown but not that either. The mystery continues...