OK- - - -it looks like the dowel is either pushed in to the "full flow" position, or it's missing altogether. The oil passage behind the plug immediately beneath that dowel is drilled into the output gallery from the oil pump. If the dowel was in the "out" position, the oil could flow upwards to the raised area that contains the front to rear oil gallery that feeds the main bearings. In its current position, the oil must flow out of the block, through some type of full flow oil filter, and enter the main oil gallery through the other large opening further back on the block. If you choose not to run an oil filter, the two large ports must be connected with either hard pipe or a 1/2" diameter flex line.
The original full flow canister was about 5 inches in diameter and close to a foot long. Those canisters are a little hard to find, and so are the filter elements. You can substitute an aftermarket filter adapter and a spin-on filter, as long as the lines to and from the block are large enough to carry enough oil to lubricate the engine. If the filter is attached to the intake manifold you can use 1/2" OD tubing and brass fittings. If the filter is mounted on the fender well, you'll need flex hoses to accommodate engine torque. Be sure to route flex hoses as far away from the exhaust pipe as possible. A heat damaged flex line will empty the oil in a few seconds.
Jerry
