Here's my hypothesis as to how the Cuckoo Sussex came to be...
Greenfire brought over the Silver Sussex and were crossing with Light Sussex to produce more chicks (which is true). The Silver Sussex began to lose their Silver Sussex appearance - started to feather out with too much white (true). Since a non-barred rooster over a barred hen will produce hens that share the appearance of Silver Sussex (true) Greenfire decided to cross one of their Sussex over a barred hen to darken up their Silver Sussex (this is a guess, but it is a common practice and has been done with Sussex in Australia and Canada). The barred gene then appeared in the chick you're friend bought from Greenfire Farms.
Just because Greenfire Farms (probably) used a barred hen somewhere along the line doesn't prevent your chickens from being "pure" Cuckoo Sussex. As long as your chickens meet the criteria for Sussex they are Sussex (chickens are not pure bred in the same way dogs or cattle are - chickens just have to meet the physical criteria for a variety).
Your chickens are beautiful by the way - I'm a big fan of barred/cuckoo birds.
, from Mabelvale, Arkansas.