I find St Mandé and Grenelle weights very similar, and look forward to the PCA article that Alpha has mentioned. But that is perhaps not too surprising, as a key person is common to the two factories – a man called Charles Bredgen. Charles’ father - Thomas - was born in Stourbridge, and was a glass maker at the French factory at Choisy-Le-Roi. His son Charles worked at St Mandé, then founded the Grenelle factory. I believe he worked in Finland for a number of years before returning to France to work at Clichy. There is an article about him by Philippe Frère in the 2003 PCA Bulletin.
I don’t find the weights very like Clichy, but that may be from years of trying to spot the differences. Here are some of the features I note for St Mandé and Grenelle: the glass is often a grey blue, and of poorer quality than Clichy. The profile of the weights – especially the miniatures – shows the sides curving under to a small base, and there is often a distinct groove towards the base where the pick-up and casing glass were joined. Often the concentric designs in the miniatures are off centre. I also find the canes less complex than Clichy.
I have added images of the top and base of a medium St Mandé concentric, a small St Mandé concentric, and a Clichy closepack to help illustrate some differences.
Hope this helps