Sorry to tell you this, but you boat is bigger than you think.
The DS21 hull is only 18ft long. Measure it over the tubes and you will find it is 20+ feet long. Boat builders tend to be a bit liberal with their measurements.
As Andy (Hightower) noticed, BWM used a tape measure which had the first foot or so missing.
The bow "moustache", huge flare and slightly stepped transom is the give away.
Your boat is a dead ringer for mine, except mine has a slightly more used look to your cracking new tubes.
It is also identical to Bill & Janets and Bajans.