Records for your property are kept in the property files at the Building Dept located at 55 Constitution Dr. Approved septic plans were not required by New Hampshire until the mid 1970's. If there are any records of those early approved systems your only chance of finding them would be to request a search of the records kept by the NH Department of Environmental Services - Subsurface division.
You need to look at your property file and see if it contains a copy of the original septic plan or if the system has been updated or changed from the original design. If your system has been repaired by way of "in-kind" permit then a copy of that permit will be in the file. If you have been told it has been replaced or repaired and there is no record in the file of the repair or replacement, the department will not verify this unless supporting documentation can be provided to the town. New Hampshire Department of Environmental Services