Need help dismantling Gaggia "Coffee" Boiler

I've had my Gaggia "Coffee" for about eight years, and it has recently
developed a problem. The water would no longer pump out through the
coffee holder, although the steam part of the unit worked fine.

I have partially dismantled the boiler and head. I was dismayed to
find a lot of junk in the boiler: it looks like brown and white
crystals, which break up when prodded. I cleaned all that out, and
have also started to clean the mating faces so that it will seal back
up again when reassembled. The rubber sealing ring is in excellent
condition, strangely enough. Here is a photo of the boiler unit before

Here is a photo of the weird gunk inside:

I'd like to disassemble the base part, shown here:

so that I can clean it out. But I cannot see how this piece comes
apart. Help!

Thanks :-)