If all you get is an occasional drop of oil on the driveway then that's not too bad for a 44 year old red block, even one with an amazingly low 100K km on it.

From your pics and videos, it's hard to guess the source, but there are of course likely candidates and there may well be multiple points where oil is weeping, including the oil pan gasket. Maybe go around and snug up the accessible oil pan bolts, but not so tight that you warp the pan lip (check the torque spec and maybe go a couple of ft-lbs higher).

Odds are high the rear main seal is one source. If there's any positive crankcase pressure then oil can push out around the engine seals. Drips of oil collecting at the opening in the bottom of the bell housing is the classic symptom of the rear main seal and if it's red oil then the trans bushing/seal is weeping. You say the valve cover gasket isn't leaking, but please double check the area below the valve cover at the back of the block. There's a half-moon plastic plug there than can sometimes weep if the valve cover gasket isn't nice and snug back there. If it is the rear main seal then consider switching to a high mileage oil. High mileage oils have seal conditioners as their main additive to help pucker a worn seal. Problem with oil conditioners is that they will also pucker the front seals, which will help them wear out a little faster, but they're relatively easy to replace, so it's usually worth the tradeoff rather than having to drop trans to get at the rear main seal. Even if they're not leaking, consider replacing all three front engine seals with every new t-belt. At your age and mileage, a seal conditioner would likely be beneficial. I'm still on the original main seals on both my red-blocks with over 400K km on them. Both have been weeping for many years, but I've managed to keep it down to a tolerable level by occasionally using a high mileage oil with a seal conditioner.

The front oil seals are usually the most obvious sign of a plugged PCV system. Oil can drip down behind the timing belt cover and get blown back under the engine and allow oily, black residue to collect all over the place, even as far
back as the bell housing. It may not be obvious that's where it's coming from.

Every time you do an oil filter change or even even when filling and spill oil on the valve cover, oil can easily drip down and collect in odd corners under the engine and around the engine mounts. If the oil pressure sensor wire gets caught behind the oil filter then it can really leak. It's also possible the oil filter isn't snugged down tight enough to make a good seal. Last oil change I did I must have had oily gloves on and didn't do a good enough job of hand tightening the filter, so got a tiny bit of weepage there that took me a couple of weeks to notice.

With yours, I'd start by doing two things:

First, clean all the oily residue under the engine as best you can. Over the coming days and weeks you'll then be able to see where the fresh oil is collecting and coming from. Use spray engine cleaner (be generous, do it on a warm engine, lots of newspapers on the ground to catch the mess), a toothbrush, wipe reasonably clean with rags/paper towels, then rinse it down with a hose and (soapy) water when you're done. Once it's dry you'll be able to notice where fresh oil is coming from. Realize that driving will blow oil backwards under the engine, especially if the belly pan is off, so check for leaks sooner rather than later.

Second is to inspect and clean the crankcase ventilation system. With the engine running, remove the oil filler cap and place a thin rubber glove (or something similar) over the filler neck. If the glove sucks down, the PCV system is working okay. If it puckers, the system is plugged and likely pushing oil out past the engine seals and even the oil pan and valve cover gaskets. If it stays flat then it may be partially plugged. In any case you should remove the main crankcase hoses and inspect the nipple areas and even blow into the ends to make sure the nipples aren't clogged.

Near as I can tell, your B19A has a hose between the air intake tube and the valve cover (middle of the top or side at the front of the cover). There's a nipple at the air intake tube you should open and check. It's not likely blocked, but there may be oily residue condensing in the air intake tube that you may want to clean out so it doesn't collect up at the carbs.

That black box you pictured is the oil separator box on the block. The nipples can get caked with oil and slowly start to plug, but at your low mileage I think that's unlikely. It would still be worth checking by removing the oil filler cap then blowing through the hose back down through the separator box to make sure it isn't blocked. You can even remove the hose from the top of the box and poke a small screwdriver or drill bit around to loosen any caking inside the top nipple. Checking the bottom port requires removing the box -not worth the effort unless you run out of ideas and suspect it may be blocked,

I'm not so sure your B19A has a flame trap like the later B20/B21/B23/B230 OHV red blocks. If not then it may have a PCV valve. I can't see either one in the B19A schematics I'm looking at. If present, they will be found inline at the top of the hose off the oil separator box. If present, remove them and inspect to make sure they're not blocked. Clean and/or replace as needed. If it happens to have the original coiled brass flametrap element, those can more easily get plugged, so replace it with a newer style plastic one with holes in it.

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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now