The Earldom of Northumbria is granted (as OTL) to Henry of Scotland by Stephen of England. Henry does not die before he accedes to the Scottish throne and thus the earldom is retained by the crown, but it is granted to a son of the king, not held by the king himself. Following the accession of Henry II to the English throne after the civil war, the Treaty of Horndean confirmed the earldom as an English fief held by the Scottish throne. The First Barons' War in England changed this. Alexander II withheld homage from Louis and on the death of John provided troops in support of Henry III. This support was recognised in the Treaty of Kingston which stated that the King of Scots owed no fealty to the King of England for the Earldom.
The status of Northumbria as English or Scottish remained unclear until the 1237 Treaty of York, which confirmed the boundary between the two countries to be the Solway Firth in the west and the mouth of the River Tees in the east (the southern boundary of Bernicia, one of the 6th/7th century predecessors to the kingdom of Northumbria). The Earldom of Northumbria became the East March of Scotland, more southerly than OTL.
As the March is held by a son of the king, the banner of arms is the king's arms with a bordure (following the Stodartcadency system); if the king has no son, the post is held vacant but still held by the Scots throne.
This particular banner of arms shows a later version of the lion rampant, surrounded by a bordure of the Northumbria colours (paly Or and gules). I don't have time to do the full achievement of arms, but it would have the same in the shield, with a royal helm and a crest with a prince's crown and a lion holding a cross (symbolising the importance of Christianity in Northumbria) and a sword, with supporters of St. Cuthbert of Lindisfarne and a unicorn (for Scotland), and a motto 'fides et vigilantia' meaning 'faith and watchfulness'.
Escutcheon Or a lion rampant within a double tressure flory counter-flory gules (for the Kingdom of Scotland), within a bordure paly Or and gules embattled counter-embattled (for Northumbria); For a Crest, upon a Royal helm a princely crown proper, a lion sejant affrontée Gules, princely crowned Or, holding in the dexter paw a cross and in the sinister paw a sword both erect and proper; Motto 'fides et vigilantia' (faith and watchfulness); Mantling Or and gules; for Supporters, dexter St Cuthbert holding a pastoral staff, sinister a unicorn Argent armed, crined and unguled Proper, gorged with a princely coronet Or a chain affixed thereto passing between the forelegs and reflexed over the back also Or.
After all that, here's the banner of arms of Robert, 11th Warder of the East March of Scotland, being the Earldom of Northumbria, held in perpetuity by the Scots throne and invested in the son of the King of Scots:
ps it might look like the rule of tincture is violated here, but that doesn't apply with bordures; it is unusual for the bordure to share one of the tinctures of the field, but not completely unheard of.
The flag of the Countship of Rethel, in a union with the Dukedom of Luxembourg, held by Prince Balthasar, the heir apparent to the Kingdom of Spain
In TTL, the Franco-Spanish war ends with an unqualified Spanish victory resulting in a number of territorial changes favoring the Spanish side. One of those changes concerns the Duchy of Luxembourg, the title of which is held by the King of Spain. Instead of losing territory (as in OTL), the Duchy of Luxembourg instead gains the County of Rethel, although under the terms that it is not annexed into the Duchy but rather held in an union with it, so that the Duke of Luxembourg is always the Count of Rethel as well.
Also in TTL, the marriage of Philip IV of Spain and Elisabeth of France is a long and happy one. Elisabeth does not die in 1644, nor does the heir apparent, Balthasar Charles die in 1646. When the Franco-Spanish war ends, Philip IV decides to bestow a royal dukedom to his heir apparent and chooses the Duchy of Luxembourg with the newly acquired County of Rethel for that. In that union, Luxembourg is seen as symbolizing steadfastness and Rethel as symbolizing victory.
The flag is quartered between the Royal Crown of Spain for the heir apparent and the arms of Luxembourg and Rethel. Both of those arms are set on fields of striped blue and white, symbolizing that the territories are held together in a union but still under Luxembourg.
The premise is essentially the March of Brandenburg not being sold off or given away by the Luxembourg family during their tenure as kings of Bohemia and Holy Roman Emperors. Their dynasty is more successful overall, and Brandenburg remains part of their domain. As the years go by, and the Holy Roman Empire faces its night inevitable (ATL) decline and collapse, Brandenburg is incorporated into the Bohemian kingdom proper as their rulers consolidate what lands they still hold. It had de facto been a Bohemian border province for centuries at this point, often held by younger sons or side-branches but always remaining closely tied to the Luxembourg monarchy proper.
Heraldic resources courtesy of the wonderful Wappenwiki
