I didn't see anything in that DG TN saying that there were any acceptable substitutes for the specified instruments on the equipment list, just how to install additional equipment.

Schleicher has a similar TN which does specifically state that you can substitute other makes and models of the required instruments so long as they meet a certification standard (TSO, JTSO, ETSO) as well as giving instructions on the installation of additional equipment. My 15B type certificate and flight manual specifies the generic equipment required but says nothing about any particular make or model. My friend's 19 on the other hand does mention specific makes and models of instruments in the manual.

Another club member with a DG800 wanted to put a Bohli compass in the glider but the type certificate and manual required a certain make and model of compass, period. Yet another ran into trouble with importing a Polish sailplane as the type certificate required a specific make and model of altimeter (different from the one that was installed) and a specific make and model of seat belt (and ONLY that one) which also had a lifetime of 15 years - and the company which manufactured the seat belts had been out of business longer than 15 years.