Some email-discussions already emerged concerning the essay Mapping Arda before the online-publication of OtherMinds 01. I will now use this public forum to summarize them for the broader public.

Why does the First-Age-map show a vastly extended Sea of Helcar?

Tolkien drew two versions of the Sea of Helcar. The first one can be found on Ambarkanta-map-IV (HoME 04: The Ambarkanta; 1936). It is well drawn in ink. It shows a tiny round Sea of Helcar at the northern centre of the continent of Middle-earth.
The second version is on the back of Ambarkanta-map-IV. This Ambarkanta-map-V (1936/37) is hastily and very sketchy drawn in pencil. It shows a very vast version of the Sea of Helcar which stretches many leagues to the east right to the foothills of the Red Mountains.

Ambarkanta-map-IV shows the situation of the landscapes of Middle-earth before the Battle of the Powers, whereas Ambarkanta-map-V shows the situation after it. Hence in 1936/37 Tolkien imagined that the Sea of Helcar was widened as a result of the Battle of the Powers. However, this idea was no longer extant in the 1950s, as can be concluded from the following quote:
 
"§38 In the changes of the world the shapes of lands and of seas have been broken and remade; rivers have not kept their courses, neither have mountains remained steadfast; and to Kuivienen there is no returning. But it is said among the Quendi that it lay far off in Middle-earth, eastward of Endo[r] (which is the midmost point) and northward; and it was a bay in the Inland Sea of Helkar. And that sea stood where aforetime the roots of the mountain of Illuin had been ere Melkor overthrew it. Many waters flowed down thither from heights in the East, and the first sound that was heard by the ears of the Elves was the sound of water flowing, and the sound of water falling over stone." (HoME 10: Annals of Aman; 1950)

Thus Cuivienen lies near the foothills of the Red Mountains. This is exactly the situation as shown on Ambarkanta-map-V. Hence in the 1950s the widening of the Sea of Helcar should have taken place in the long Ages of the Stars *before* the Battle of the Powers. This was a new development and contradicts the Ambarkanta (1936), which says that the Sea of Helkar widened *with/after* the Battle of the Powers.

1936 (Ambarkanta)
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a) Fall of Illuin ==> Small Sea of Helkar (Ambarkanta-map-IV)
b) Awakening of the Elves ==> Cuivienen as a little Bay at the east-end of the small Sea of Helkar (Ambarkanta-map-IV)
c) Battle of the Powers ==> Widening of the Sea of Helcar to the foothills of the Red Mountains (Ambarkanta-map-V)

1950 (Annals of Aman)
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a) Fall of Illuin ==> Sea of Helcar of unknown extension (perhaps still as on Ambarkanta-map-IV)
b) Long Ages of the Stars ==> Perhaps widening of the Sea of Helcar to the foothills of the Red Mountains (as on Ambarkanta-map-V)
c) Awakening of the Elves ==> Cuivienen as a little Bay at the east-end of the wide Sea of Helcar at the foothills of the Red Mountains (as on Ambarkanta-map-V)