The Copa Del Rey did not always have the format it has today. When it was founded in 1903 it consisted of champions and runner-ups of regional leagues. In the late 1920's Spanish soccer reorganized to have a national league along the lines of what we see today, and therefore the Copa Del Rey also had to be restructured both in terms of format and qualification method.
In May of 1927 Real Union won the last Copa Del Rey in the old format defeating Arenas Club in the final by 1-0. Nevertheless the Spanish Federation wanted to give the "old" cup a proper sendoff over the summer before it entered its' new era. Motherwell, from Scotland, and Swansea City, from Wales, were invited as guests in this celebratory unofficial Copa Del Rey.
Motherwell and Swansea faced in each in a sort of semi-final to determine who would play against Real Madrid in the final. The match was historic, it was the first time two British clubs had ever played against each other in Spanish soil, and Motherwell took the victory by 4-3. King Alfonso XIII called the match a "brilliant display of scientific soccer".
Motherwell still had quite a mountain to climb, the scale seemed tilted for the home team to win. Not only did Real Madrid have the advantage to play at home but also was allowed to temporarily borrow players from other clubs just for that match. The Scotts were also only given 48 hours to rest from the semi while Real Madrid was fully rested.
Despite all odds Motherwell put on a show and defeated Real Madrid by 3-1. They were awarded a special version of the Copa Del Rey trophy that the King himself had donated for the occasion.
Not satisfied with their achievement Motherwell went on to win the Barcelona Cup, defeating Barcelona and Swansea (again), as well as 3 other friendly matches around Spain and France.
Out of all that happened during that magical summer of 1927 for Motherwell, the unofficial Copa Del Rey title was by far the most memorable. An achievement that the steelmen are proud of to this day.