I did some research of my own and here is what I came up with.
The 170Va parts manual shows the light (with an on/off switch) to be oval in shape and it bears the part # 136 825 00 01, whereas the part # of the plastic cover is 136 825 00 10.
I found a picture of a 170Va ceiling on the net and it also shows the light to be oval.
The third picture is from a parts lot which was for sale. The light in that pic is supposed to be from a 170Va and is also oval, but it's plastic cover is not plain but has a parallel straight lines pattern. This light is very similar to the one that is presently on ebay the link of which I have attached.
Intrestingly there is another dome light put on German ebay today ( refer to link below) which is also supposed to be for a 170 V and D and it looks very similar to the one that is fitted on John Ellis's 170Vb.
http://www.ebay.de/itm/Mercedes-Oldtimer-Chrom-Innenleuchte-170-V-D-Ponton-180-190-220-S-SE-SL-Adenauer-/121177613270?pt=DE_Autoteile&hash=item1c36bfa3d6 A note worthy thing is that this dome light is without a switch whereas the 170 Va parts manual light description indicates the light to be with a switch (item# 48 Deckenlampe mit eingebautem Schalter).
Therefore what I can conclude is that the 170Va's light was definitely oval with an On/Off switch, whereas the 170Vb's light was probably the one in the pic that John posted and did not have a switch.
Can you gentlemen shed some light.
