Saint Constantine the Great witnessed a great sign on the sky – he saw with his own eyes in the heavens and a trophy of the cross arising from the light of the sun, carrying the message, “In Hoc Signo Vinces” (“with this sign, you shall defeat”) before the decisive battle with Maxentius. After this vision, among other things, Saint Constantine made three crosses and positioned them in the frontline of his army at the Battle of the Milvian Bridge in 312.
One of the three crosses – the only one which survived till today – is the one from the image. So, this form of the cross isn’t the Russian cross or Byzantine cross because it existed before even the „Byzantine empire” which is in fact a conventional period of the Roman empire which started years after this cross appeared.
The rounded bigger accessories are added on a later date, the small filigran is Byzantine.