I’ve now got some first hand information that answered most of my original questions, so I have edited this post for the information of others.
The PTL omega 12.7 x 99 is a match grade round available with a solid brass 800 grain bullet or an Armour Piercing bullet, made by Primetake Limited in England, UK. and developed for the RPA Rangemaster .50 Tactical Rifle.
Rangemaster Precision Arms Ltd, supply PTL Omega match ammunition with the PTL omega headstamp, not the RPA headstamp.
The RPA headstamped ammunition was made in a very small amount (quantity not known) for a specific procurement demonstration of the RPA rifle in late 2007. This was supplied to Rangemaster Precision Arms Ltd by Primetake Ltd.
I don’t yet know why some of the spent cases seen have red case-head overpainting. If anyone knows why, the information would be appreciated.