Hi Librarian,
Just seen this today with Keith. A quick answer is that the Lombards were used as mercenary cavalry by the Byzantines at least as early as the battle of Taginae in 552. They were known to the Romans as early as the 1st century AD and were considered part of the Suebian peoples, but early Roman histories are perhaps not the most reliable source of information. It's not exactly clear to modern historians where exactly they originated but definitely a western Roman tribe. The earliest Lombard histories, written in the early medieval period suggest they migrated from southern Scandinavia and basically headed east during the migration period. They ended up in modern Hungary. They had fought both Heruls and Gepids in their travels, both tribes well known and used as mercenaries by the Byzantines. It must have been when they were in Hungary that Narses brought them to Italy as mercenaries. At the battle of Taginae he had them dismount to stiffen the infantry. They seem to have liked the climate as they invaded it along with their allies and subjects in 568.
Plenty of opportunities for 'cross over' of equipment in both directions during this period and I'm sure the Byzantines would have no problem trading with Lombards or taking equipment from dead allies.
I hope this answers your question. Adam