If you're an artilleryman at The Patrol and wearing a Mounted Services Jacket, you should have a pair of Shoulder Scales (or at least the attachment hardware).  These scales are nice copies of the originals and even include the oilcloth liner.  A set of complete scales comes with the attachment hardware, or you can just order the attachment hardware on its own.  All will be shipped to you in time to stich the hardware onto your jacket before the event, or to the vendor making you a jacket if you also order one of those.

Shoulder Scales - The Patrol