Hook wrench with tip  80/90 mm

Special steel, hardened and treated with anti corrosion.
Hook wrench according to specs  DIN 1810 A.
For use with nuts like on the 928 schocks:  DIN 1804.

Motion Pro
SHOCK SPANNER WRENCH
•Heat treated alloy steel
•Fits many street, offroad and ATV single shock applications
•Fits Honda ATC 200/250R/350X
•Fits 68 and 87 mm lock rings with a 8 mm notch

Motion Pro Single Shank Nut Spanner 08-029. Available at motorcycle and RV
shops for under $20. Spray plenty of WD-40, LPS or other suitable
"loosening agent" on the threads and try to clean as much crud off them
before you start trying to turn the nut. Keep bandaids on hand for the
inevitable slip of the wrench.
http://www.motionpro.com/motorcycle/partno/08-0029 

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Grainger
PROTO C474 Spanner Tool
Spanner Wrench, Adj Hook, 4 1/2-6 3/8
Adjustable Hook Spanner Wrench, Length 12 Inches, 4 1/2 to 6 3/8 Inches Diameter Capacity
http://www.grainger.com/Grainger/items/4DC29?Pid=search

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Vicegrip model 11R
Has great clamping power to reduce slipping risk.

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Britool 3152 51-121mm C Hook Wrench C Spanner
C-hook wrenches are specially designed for use with slotted retaining rings. The hinge facility allows for use on a range of diameters for greater flexibility. Capacity. 2 - 4.3/4in. 50-120mm
http://www.thetoolacademy.com/product_detail.asp?productId=544