Thanks for the idea, it may come in handy.
To elaborate on the point for those following along, the E61 dispersion screen is held in place by the gasket. The gasket snugly fits in the groove of the brewhead; the thread How to remove the dispersion screen and gasket of an E61?
covers it in more detail. Some have suggested using brew pressure to force the gasket out (i.e., snug the portafilter slightly, pressurize the group to force water behind the gasket, loosen the portafilter slightly; repeat until the gasket pops out).
There's lots of threads about removing
the gasket, but I think this is the first about putting it back
. It also helps if you use the portafilter without the basket to initially seat the gasket. That is, put the dispersion screen and gasket in the basket-less portafilter, slot them into the grouphead, and then pull up until the gasket is deep enough in the slot to engage the portafilter ears in the grouphead channel. Once the ears engage, snug the gasket in a bit, then remove the portafilter and drop in a basket. The gasket should be down far enough in its groove to seat it the rest of the way using the tightening of the portafilter.