Any idea what the thread size is that retains the valve?
You can obtain screw fit schraeder's as per the pic.
The larger thread measures at 10.1 (SS) & 10.3mm (ALU) OD but same thread.
If in UK could post you an aluminium one if it looks likely - would need cutting down etc.
You could be onto something here.... possibly the rear shock is goosed and these are actually gas struts - no luggage is ever carried, they just needed additional mounting points!
The 500 / 502 / 507 Alpina's I've had have all been 1" but I also have a 550 Alpina with 1 1/8" but this is a later V brake model with no rear canti bridge. Think you should find similar correlation.
Okay, thanks for clarifying and additional pics.
Not one for me I'm afraid but you might want to weigh it without the headset fitted and list the fork weight and if possible measure the axle to crown height and shares this. Most people will find this helpful.