I believe the only difference is the colour of the base.
They were all made by Mullard UK, which was owned by Philips, Oskari knows much more than me.
When you know how to read the codes, you'll find out which factory made them, if they are correct of course!
This thread has more info: http://www.head-fi.org/t/558352/ecc33-ecc35-tube-addicts
There are short base ones brown or black, with D-Type wire or inverted cup getters, grey and black plates too.