As per today's patch notes
"Siegemaster Costume: Obtained by consuming a “Runebox: Siegemaster's Costume”.
This Runebox is obtained by using a Siege of Cyrodiil Recommendation, which consumes 50 Siege of Cyrodiil Merits.
The Siege of Cyrodiil Recommendation can be purchased from Tel Var General Merchants for 1,000 Tel Var, and the Siege of Cyrodiil Merits are obtained from completing Imperial City Daily Quests. "
The only available Siege of Cyrodiil Recommendation is available for 100 Tel Var and only reflects the usage of combining 20 into the Runebox: Siegemaster's Helm

Popped on over to pts to see if there was any way to check before using the Siege of Cyrodiil Recommendation on live and they do not have an Imperial City server up to check so I am unable to see if there is a distinctly different one. If there is, its not showing up in the Tel Var General Merchant (checked with 2 characters) and if it is NOT distinct from the one that was already on live for when the helm was the only one available, then the patch notes are incorrect (only costs 100 telvar instead of 1000) and the text description is incorrect.