The three dipsticks and tubes:
Top: 18G to GF, sliding dust seal, no stop. These rest on the sump bottom to give the correct oil quantity.
Middle: 18GG to GK and early 18V581/582/583, 584/585 and 672/673. These have a stop on the stick and need the correct tube to ensure the correct oil quantity. the tube screws into the crankcase. The stop effectively wedges into the tube to give a decent seal on engines with positive crankcase ventilation, where the crankcase is under vacuum all the time the engine is running.
Bottom: Later 18V. These also have a stop and need the correct tube to ensure the correct oil quantity. The tube is pressed in to the crankcase.
The middle and bottom sticks are said to be interchangeable, but the tubes are significantly different lengths which will give significantly different oil quantity, more with what is indicated to be the later tube in this picture. This goes against the smaller quantity of oil that other sources indicate was used in the later engines, unless the longer pressed-in tube goes in much further than the shorter screwed tube, which seems unlikely. I have the straight stick in my 18V673, which would be correct, and comparing the exposed length of my tube and stick, with the sticks and tubes here, with an allowance for how much might be in the block, my tube is much closer to the long tube. Which still leaves the question about oil quantity on engines with the shorter tube.
(image from British Automotive)
18G, GA to GF, and early 18GG and GK engines had the sump plug forwards of the right rear corner, sumps 12H395 (18G and GA) and 12H1426. When used with the early dipstick without the stop the stick rests on a raised portion of the sump (arrowed).
After that the sump plug was on the corner on sump 12H3541 ...
... as that part of the sump was 'cut away' to access the torque converter bolts for the automatic gearbox via the hole in the engine back-plate. (Chris at Octarine Services)
The bolt pattern on the 3-bearing sump is different to that of the two (interchangeable) 5-bearing sumps - the 3-bearing has an extra bolt along the left-hand side which moves the front left-hand one (lower left in these images) round the corner a little way. (Chris at Octarine Services)
And what is the purpose of the vertical 'tube' in the lower right-hand corer of the 5-bearing sump, with matching hole in the 3-bearing gasket? There is a matching hole in the block right by one of the sump screws, overflow from the relief valve?