There's not much in Honda frame numbers (VINs) from that era, but here's the story:
Model: CB175 (but you knew that, right)
Year: '6' signifies the 1971 model year (5 for 1970, 7 for 1972) (see CycleChaos for details
Production Sequence: 3251 is the production sequence, although I'm not sure if Honda started at 00001 or not. Other manufacturers did not.
So, your 1971 Honda CB175 was built in November, 1970.
I have a similar situation, where my '72 CB175 was built in October 1971.