Middle mouse button (2) not recognized on modular X11

Hi,

I just got an old-school three-button Logitech Trackman Marble (PS/2) 
for use on a netbsd-9 modular X11 setup. Thing is - try as I may, I 
cannot get X11 to recognize button 2.

The relevant bits of the installation are

% dmesg | grep pms
[     1.004071] pms0 at pckbc1 (aux slot)
[     1.004071] wsmouse0 at pms0 mux 0
% pkg_info | grep xf86
libXxf86vm-1.1.4nb1 Library for the XFree86-VidMode X extension
libXxf86dga-1.1.5   Library for the XFree86-DGA X extension
xf86-input-mouse-1.9.3 Modular Xorg mouse driver
xf86-video-intel-0nb20201027 Modular Xorg Intel video driver
xf86dga-1.0.3       Simple test client for the XFree86-DGA extension
xf86-input-ws-1.3.0.20151010nb2 OpenBSD's wscons input driver for 
modular X.org
xf86-input-keyboard-1.9.0nb3 Modular Xorg keyboard driver
%

I tried both 'mouse' and 'ws' drivers with the same negative result. 
xev(1) doesn't show anything for the middle button, nor does debug 
output for ws(4).

Here's the rub: This trackball works just fine on an old thinkpad with 
netbsd-5, all buttons doing what they are meant to. And I have the same 
trackball @work three-buttoning fine with native X11.

Any ideas?

Cheerio,
Hauke

-- 
Hauke Fath                        <[email protected]>
Linnéweg 7
64342 Seeheim-Jugenheim
Germany