You need a single-float Buchholz relay of type 03 (AF 25/10) with one cable gland of M20 x 1.5 and one dummy plug. The damper should respond at a insulating liquid flow of 1.00 m/s. The switching system should be equipped with three normally-open contacts (magnet contact tubes). The device should be delivered in colour RAL 7033.
Accordingly the relay ordered has the following

type code: 03-1.44.99.-0209
 

Explanation:
03 = Single-float Buchholz relay type 03 (AF 25/10)
1 = M20x1,5: 1 cable gland and 1 dummy plug
44 = Casing colour RAL 7033 (cement-grey)
99 = Switching system equipped with three magnet contact tubes
02 = Damper setting: 1,00 m/s ± 15 %
0 = Not occupied
9 = Contact setting of switching system: 3 normally-open contacts