Hi,Rob looks like a V21 Watsonian chassis.. http://sysidecarclub.co.uk/node/69
I have a V23 chassis nearly ready to fit to one of my M20s. According to above link they were made until 1948 so could be a period fitting for our M20s.
The one on Ebay has the front mounting springs replaced by what looks like rubber blocks.The V23 chassis I have has quick release mountings for the body just pull out front and rear pins.Overall these are a nice lightweight chassis which is also strong..Dave