This month Thomas Mayer takes us on the historical journey and reveals one of the very first indirectly heated triodes which saw a wider commercial use. The 27 is the ancestor of a long row of low to medium mu triodes coming after 27 introduction.
27 was introduced in the golden age twenties of the last century. Originally it was named UY-227, but later downscaled to the to the two digit numbering.
Link: here