This model of heating element gasket, which is triangular with rounded off edges is typically for horizontal heating elements used with La Spaziale espresso machines. 

This specific model is made from polytetrafluoroethylene(PTFE) there are also versions available from fibre gasket material or just a flat round gasket. 

Normally La Spaziale uses o rings to seal off the heating elements. 

The La Spaziale code for this is: 02418

This specific heating element gasket has the outer dimensions of 85x85mm with a gasket thickness of 2mm. The internal hole has a diameter of 55mm. The gasket has also 3 mounting holes, these holes have a distance of 63mm in-between.

You can use this PTFE gasket on a variety of heating elements and with a couple of models like the La Spaziale S40, la Spaziale S8, La Spaziale S9, La Spaziale S3, La Spaziale S2 and the La Spaziale seletron plus.