The 6604 and the 13139 are not the same fitting. There were two different versions of the bulb-holder in the V35, due to the original bulb being discontinued by Philips. There is a handy diagram and details at this link. Also note that there are a couple of different versions of 13139, with different manufacturers and very different expected service lives.
Originally Posted by BMbikerider
I looked on my previous supplier for Philips 13139 and they are "no longer available - last stock" on the website. This mod, by the OP, could be very useful in the future.