I looked through my parts bills... I think these are the correct
numbers.

Quant    PN                    Desc.
1        251-201-147-B   This is the tube and fittings that straddle the
tank  ($27.78)
1        251-201-139-A    This is the gas tank filler neck gasket
($6.22)
3        N-901-004-01    These are the small vent line gaskets ($2.88)

This was for an '88 Westfalia