Yeah, this probably is a Silca article 24.4 chuck or pump head, they varied a bit in details over the years but doesn't seem to be something the NEW Silca owners offer in their catalog (in stainless steel let alone brass). But as BG says the o-ring is probably something you'll find a workable replacement for in a hardware store with a good plumbing section. It may be originally metric but chances are you can find a good enough match.
TIP: daubing a little silicone grease on neoprene parts makes them last a lot longer but don't go overboard with the grease!