Yikes I think I have this problem. On advrider I have been telling the sorry tale for the past while. Piece by piece I have been replacing parts, conrod bearings, pistons, barrels, piston pins, rocker arms... then onto the timing chain that many had assumed caused the noise. But now I believe I did find a pin a few years back but without problem ... but that assumption has come back to roost with heavy bottom end knocking that must be the bushing behind the bearing holder at the front of crank.
Great pix but what is it ... any further pix would be very helpful since I must work in a cold 0 degree garage
Here is a youtube video of the problem. https://www.youtube.com/watch?v=BYNd_dX6rDU
My pic above is of the front main bearing housing. The protrusion you can see in the inner circular section is where the dowel locates. As the dowel had fallen out and the bearing had rotated, you can see in the pic that the protrusion no longer lines up with the hole in the bearing into which the dowel would have originally fitted. So the diagnosis of this problem is that a) you find a dowel in the sump (not in the clutch housing) and b) the hole in the main bearing doesn't line up with the housing for the dowel any more (if it does line up but there is no dowel, you have been lucky and you should replace it). Symptoms will include anything that can be caused by oil starvation on the high pressure oil circuit. The starvation happens because as the bearing rotates it also ceases to line up with the various drilled oilways in the housing.