Originally Oakeshott Type Xa swords were actually classified as type XI swords, but in the end Oakeshott felt that they deserved their own subcategory as they were too close to type X to fit within the Type XI category, but the narrower and deeper fuller could not be ignored.
But of course, it was not just the fuller that guided his decision, but their placement in the historical context, as all extant examples dated from the 11th to the 14th century, while type X started and finished two centuries earlier, from the 9th to the 12th centuries.
Like the parent group type X, these were a transitional sword - similar in shape and style to the Viking swords that it evolved from - and was a stepping stone to the Type XI cavalry swords which shared the same thin fuller, but had longer, more slender blades better suited to mounted combat.
Type Xa was in use for a much longer period than the Type XI cavalry swords, and so while the thinner fuller may at first glance appear insignificant - in reality it marked a serious departure point from the Viking era swords and were used by late period Vikings, Normans, Anglo-Saxons, Crusaders and Templars before eventually falling out of favor in the 14th century as they were quite ineffective against the increasing use of plate armor on the battlefield.
Despite spanning a period almost as long as the Type X (which was used for the longest period of any one sword type in medieval history) replicas of this type are rarer than Type X.
The very best and most historically accuate replicas are made by Albion Swords who have no less than 5 Type Xa replicas - one of which is presented below along side two budget entry level options.
MyArmory has an excellent in depth article on Type X swords here, which also includes a little side note on Oakeshott Type Xa swords, though has quite a few images of antique Xa swords worthy of perusal.
And of course, you can - and should - read more about these swords and all the others in the Oakeshott Typology in his book, Records of the Medieval Sword
I hope this information on Oakeshott Type Xa swords has been helpful. To return to Oakeshott Typology Made Easy from Oakeshott Type Xa Swords, click here