Excerpt from the rules of cribbage:

There is a special award for the 5-card straight flush (but, not, interestingly, the 4-card variety) of 5 points, bringing the total to 15.

In a pair royal each card is 'worth' 3 points of the total 12. Therefore each card in a 5-card straight flush should also be 'worth' 3 points, making 15.

The 3-4-5-6-7 flush contains two 15-2's in addition to the straight flush 15, making a resounding 19. Ihis causes scoring confusion, though. Perhaps this score should be announced as "I really have nineteen. No, honestly."

Mark - from what I can gather any player can score 19 in American cribbage. In the UK a player can only score for the flush with his own 4 cards. Only the crib can score 19. Statistically its the same chance as 29. I have never seen anyone get 29 twice - the odds of once are over 200,000 to 1.