Interesting indeed. My 1676 still have the 14mm bolt for the oil filter. Though a finned one was fitted when I could get hold of one.
The smooth round one is one of the few things to have disappeared for me over the years.
Not a word about them holes in clutch centre we hear about now a days to do some extra ones of to give a better gear change?
I believe someone in this forum stated, some time ago, there were none at all in the very early CB750!
And I had in the day gear change problems. Dealer had no idea what to do.
I was the first in town to have a CB750, even the first to ask for one when the news about such a model was in the making.     
I wrote to Japan for help.
Still have a polite post card dated Oct.9´69 telling me to turn to the dealer of my area, he had been informed.
Though not a word about what he had been informed!
But I never returned for any regular service after the obligatory ones from new.
Erling.

Mine are hex 12 too and 14 mm on the 10mm bolt.
Erling.

Preservative oil out of engine and oil tank.
10 40 oil in before 5 minutes warm up. No mention of priming oil pump!
Erling.

Sorry, never noticed that date, but it is like Steve Swan says 69. 5.
Erling.

Never heard the oil filter house, or for that matters sake the 2 to 3 hole air filter case were service replacements parts, just like carburettor slides! My air filter is still a severe cracked 2 hole. Kept together by broad red tape. Come spring it will be 47 years. Had at the time gone less than 8000 km.
Erling.

Maybe the original weaker springs might help?
I only used the stronger ones for 5 minutes!
On my 90S BMW Dell Orto carbs I used to sort of tie together some turns of the spring.
Think I ended up with 4 tied together and it still managed to shut the carbs.
And I know of the handlebar rubber been pressed too hard in.
Erling.

Nice to se only one slide was affected at a time.
As i experienced it.
A danish book a decade ago stated all four could stay fully open!
Never really made any notice if it was the same one to jump the nipple.
Erling.

Probably should not ad here as I do not know about pictures!
But frame 1676 do have rear dampers stamped 4H though right half of "H" is missing , on both.
Easy to find as it has had Konis on for the past 45 years.
The samurai sword looks look more like a tilted 4 in the shape of this one as the cross beam looks moved to the left!
Upper crank case says 5-19 lower 5-20. On E vin 1638.
Oiltank 5 2 3
Petrol 4.6 2
Rear hub is in the Seeley frame can't see for now.
Erling.