There is a bulletin on EMC powerlink that clarifies their 
'NetWorker Version Compatibility' it is ESG 83215.

It says that NW 7.6 supports clients of 7.3, 7.4 and 7.5

In fact it even states a NW 7.6 server with a 7.3 SP4 storage node is supported.

Therefore you could go with a 7.3 SP4 client on Solaris 8, and it would be 
supported.   See ESG83215 for more info - for 'support' info.

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