Spoke with Makita customer service. Guy confirmed that the XRU09 trimmer has a M10x1.25LH thread.
He said that the original head was a 197296-3 like
@engineer2 bought and not the B-05119 like I suspected.
Regardless, both of those heads are discontinued and individual parts no longer available.
The replacement head is 191R00-0 which claims to be an easy one to install new line. Align the triangle pointers, slip a measured length straight thru, find the middle of the string and make sure it’s centered in the head, then wind the knob.
Reviews on line are mixed. Found one at the orange box store for slightly less than $30.
Wife weed whacks, so I will patiently wait for her report.
Customer service guy said any head that has that thread form would work. Surprised he volunteered that.