can be bored or otherwise appropriately machined without problems using conventional boring punching or electron beam boring technology as a result of their significantly greater diameter the throughbores or throughholes 9 of the inner support body 2 can be bored without problems using conventional boring techniques even when the wall thickness is 2 mm or more as is also apparent from fig2 and 3 the holes or perforations 8 are arranged coaxially with respect to the throughbores or throughholes 9 although this is not absolutely necessary it is possible to use significantly larger throughbores or throughholes 9 which do not have the same gauge as the holes or perforations 8 the term 34 gauge 34 refers in this disclosure to the spacing of the hole centers furthermore the sum of all cross sectional areas of the holes or perforations 9 in the inner support body 2 represents a greater proportion of the total surface of the perforated region a than the sum of all cross sectional areas of the holes or perforations 8 of the outer perforated body 3 in addition it can be seen from fig4 that the holes or perforations 8 in the outer perforated body 3 do not necessarily have to have a round section other hole forms or configurations are possible.