[ubuntu] Ubuntu 16.10 virtualbox guest additions 5.1.6 removal from standard installation

Installed Ubuntu 16.10 Desktop 64bit as a guest in Virtualbox 5.1.8.
After standard installation from iso with auto updates from network, kernel has Virtualbox 5.1.6 guest additions modules installed:

modinfo vboxguest
filename: /lib/modules/4.8.0-26-generic/kernel/ubuntu/vbox/vboxguest/vboxguest.ko
version: 5.1.6_Ubuntu r110634
license: GPL
description: Oracle VM VirtualBox Guest Additions for Linux Module
author: Oracle Corporation
srcversion: D95A5FA0DB511CEC415F6E7
alias: pci:v000080EEd0000CAFEsv00000000sd00000000bc*sc*i*
depends:
intree: Y
vermagic: 4.8.0-26-generic SMP mod_unload modversions

Note that this is some special Ubuntu kernel flavour. It is NOT from any package listed in Synaptic / apt.

It is also impossible to update this to 5.1.8 using the Virtualbox guest additions iso image. The installer from the iso will report success, but the mod remains at 5.1.6.
Hence some functionality is lost, due to version mismatch.

I could not find any hint on how to remove the 5.1.6_Ubuntu mod from the kernel.

Anybody got a clue on how to remove this?