The later 70s era Fenders could vary widely with weight and quality control. There were a spate of funky necks in the mid-70s. Be sure to try before you buy. The 71 should be a little lighter. The spaghetti script on the logos of the 60s was gone, replaced by the block lettering.
That being said, it's a pretty consistent and goof-proof design. Other than the weight and some of the neck inconsistencies, P basses from that era are consistent beasts.
The real magic was in the 59-65 basses, but of course that's why they cost an arm and a leg!
Here's a link to the Fender Forum that details some of the neck woes of mid 70s P basses:
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