If you look up an '83 Fiat X1/9 on the Baldwin site they list BT5 for the oil filter. If you look up '84 Bertone X1/9 they list B34. I have been using BT5 on my '85. Longer than the B34 by 1.6 mm and larger diameter by 14.3 mm, but fits just fine.I love Baldwin. B34 works great for an X1/9.
WIX lists the correct filter as 51189. Why do you use the 51056 or 51058?