That one has never had a filter installed. The small fitting is for the oil pressure gauge connection. If the dowel pin near the oil pan rail was pushed in, the block would be set up for a full flow filter with large pipe fittings. Since both the large and small diameter return ports near the oil pan are plugged, the engine was never equipped with a filter. You'll need to decide whether you want a bypass filter or a full flow, and plumb the block accordingly. Bypass filters do a better job of straining small particles out of the oil so the ideal situation would have both types of filter.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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