This is apparently a fairly popular modification where you cut the top off of E36 m3 struts, remove the guts, drill a hole in the bottom and insert a Koni Sport insert. Not only E36, but many other cars where Koni has suitable inserts
https://www.youtube.com/watch?v=PXSnNZt3NiI
Just for info some DIY links:
DIY http://syclone.motocrew.com/E36/suspension.htm
http://www.unofficialbmw.com/images/KoniProcedure.pdf

I should have known that earlier, actually I replaced 2 years ago both my original Sachs-Boge E36 M3 front struts with
312253 Sachs-Boge OEM - Strut Assembly; Front Left; OE Version
312252 Sachs-Boge OEM - Strut Assembly; Front Right; OE Version

and 'unfortunately' scrapped the old ones as both were leaking at the top with less than 50k miles, could have bought Koni sports inserts