[jonzcustomshop]

aerotruk63 helped me out with this a month ago, here is some good info:
Group 7.523 SHACKLE, REAR SPRING
2...3896376...55-68 K10-K910-K20-K920-C30-C930....INCLUDES BUSHING
/SUPERCEDES 3725531/ ( means old part # )
930 is Canadian built GMC.

http://www.parts123.com/index/google...HG-3896376.htm

http://www.autopartsobsolete.com/193...s-3725531.html

http://www.collectorsautosupply.com/...ar=36&section=

http://www.nosgm.com/query.asp?secti...+and+GMC+Truck

https://www.mail-archive.com/old-che.../msg01101.html

Took the casting number from the picture. 3656670
Does yours have a casting number?

http://www.nosgm.com/query.asp?secti...+and+GMC+Truck

http://g503.com/forums/viewtopic.php?f=25&t=238886

http://www.bizrate.com/gm-1951-gmc-t...610722859/shop