I just realized that the Master Public Keys of the three wallets I just created with Electrum 2.3.2 all start with "xpub661MyMwAqRbc...".
"xpub" certainly is a marker for identifying the String as a Master Public Key. However, do the other coinciding letters have to be always the same or do they result from a lack of entropy in the generation process?
Check the private keys to be sure.
Where do I find the private master key?
In any case: Either every master public key has to begin with "xpub661MyMwAqRbc..." or there is some lack of entropy when generating the seed. In the latter case this would be an issue with Electrum. Other programs require the user to move the mouse or hit keys on the keyboard in order to supply entropy; Electrum on the other hand generates the seed without user interaction. So, I wonder how Electrum ensures a truly random seed.
To my knowledge, you can't export the master private key from Electrum. You can always recover from the seed generated when you created the wallet though.
Also, about the master public key bit, mine doesn't start with "xpub661MyMwAqRbc", but it was created in an older Electrum version so that could be why.