I agree it would have to be a very massive internal leak to cause that low of pressure on start up. Or could the pickup tube for the pump be partially clogged or too close to the pan bottom?

If bearing clearances were too great, looks like something would be making noise. Does this engine have an external oil cooler? If not, look at the adapter for the spin on oil filter and make sure the relief valve and spring are still in place.

Check the main bearings to see if perhaps a bearing half might be missing. It still looks to me like it would make noise without one shell in place, but it might not. But with one shell missing it wouldn't pressure up very well.

Good luck in your hunting and let us know what you find.