This is a plant I was given about about 12 years ago by a friend who hybridized hydrangeas as a hobby. This was one he was unsure of what he had crossed (it was a wee plant at the time and he gave it to me to "see what it does"). In recent years, it has bloomed with a majority of plant bearing these beautiful, serrated sepal flowers. In the past few years, a back (smaller) portion has been blooming a pure white with no scallops/serrations. I assume this reflects one of the parents. I absolutely love the dainty lavender blue flowers. Any thoughts what it might be (or have been a cross between, since I am quite sure it was one of his hybrid experiments)? I'm attaching pics of the both (taken from the same plant on the same day).

Also... what is the best way to propagate the lavender portion of the shrub (to try to be sure to GET the lavender flowers and not the white)?

Many thanks in advance!