An ordinary 12v 30/40 amp relay such as this one
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=330-073&utm_source=googleps would work. Would think any number of European parts stores should carry the Bosch or equivalent type. If you want one that looks original, reproductions are available from Fifth Avenue Garage
http://www.fifthaveinternetgarage.com/parts/parts_counter_3.html but they are not inexpensive.
Kev drew a schematic using one of the same type Bosch relays for fog lights.
https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=25761#forumpost25761 The overdrive would use it the same way.
In place of the lights would be the 12ga solenoid wire on 87. The fused 12ga battery source would still go to 30. The 16ga wire from ignition switch would go to 86 and the 16ga kickdown and governor wire would go to 85. 87a would not be used.
It works like the original relay except it doesn't have a fuse on the side so you do need to add an inline fuse. When ign switch is on, power is available to one side of relay coil at 86. When speed is at OD point, the governor closes and supplies ground thru the kickdown switch at 85 to bring in the relay. Relay connects battery to solenoid via 30 to 87 and OD is ready to engage when you let your foot off. When you want to drop out of OD, the kickdown switch interrupts the ground from governor and relay drops out, letting the solenoid drop out.
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