I have attempted to correlate the mares with those listed in the General Stud Book as "dying young', a euphemism used for exportation.  The mares would presumably be at least three years old and due to the lack of any racing or produce records in England, not more than five years old.  *Roxana was still alive in 1773 and so was probably foaled no earlier than 1750.  *Express was still alive in 1764.

There are two fillies listed in the GSB who might be *Express and *Roxana:

1752 filly x Orion - Crab mare 1746 g - Flying Childers mare 1730 - Confederate Filly (Family 61)

1751 brown filly x Tortoise - Miss Vixen 1737 x Fox Cub - Honywood’s Grey Mare 1728 x Young True Blue - Sister to Little George x Curwen Bay Barb - Darcy Chestnut Arab - Whiteshirt - Montagu Mare (Family 12).

To match the mares to the cited fillies one only has longevity to compare as colors were not recorded for the Orion filly, *Express or *Roxana.  The Crab mare 1746 lived thirteen years, to 1759.  Miss Vixen lived twenty-eight years, to 1765.  One could tentatively assign *Roxana to Miss Vixen (Family 12) and *Express to Crab mare 1746 (Family 61).

Each imported mare foaled a filly in 1757 to unnamed horses.  One filly was named *Calista, the other *Pocahontas, but no record states which mare foaled which filly.  *Calista was a noted racer in North America.  Presuming a family aptitude of running fillies one might link *Calista  to *Roxana - Miss Vixen - Honywood’s Grey Mare, a noted runner in England.  Further, since the Miss Vixen’s filly x Tortoise was in Lord Gower’s possession, one might presume she was bred to one of his stallions prior to her exportation. Knowing that certain inbreeding was done, one could propose the Gower Stallion, 1740 b x Godolphin Arab - sister to Tortoise 1735 x Whitefoot - Little Hartley Mare.  *Calista’s possible pedigree follows.
 

*Calista produced three foals, a filly by Byrd’s *Valiant, a filly by Symme’s *Vampire and Dandridge’s Fearnought by *Fearnought.  The *Valiant filly produced a *Jolly Roger filly, dam of Kitty Fisher x Wormeley’s King Herod.  The *Vampire mare produced Grey Diomed’s dam, a filly by Sloe.
 

*Pocahontas was not recorded as racing in North America, the only note being she was 'of the Arabian'.  As the Crab mare was bred by Mr. Panton in England and the Orion filly was her first foal, it is possible the cover was by Panton’s Arab.  The proposed pedigree of *Pocahontas follows.
 

*Pocahontas is known to have produced one foal, a filly x *Traveller.  The *Traveller filly in turn had two fillies, one by Leeds and one by Tayloe’s Nonpareil II.  The Leeds filly foaled Taylor’s Arion, a colt by Polyphemus.  Is the name Arion reminiscent of Orion?

In conclusion, more information is required to positively identify the two mares imported by Byrd.