I posted about Nippin's Dimension Polyant sleeping bags some time ago but here's the link:
https://sv31.bestsystems.net/~danau000/web_shop.html
If you click on the 5th major link on the left menu – シュラフ – you'll get to the sleeping bags page – the Dimension Polyant ones are the 5th (blue) and 7th (green).
The green one has roughly the same dimensions as a WM Megalite but 400 grams of 95/5 down. The quality of down isn't stated but is probably 800+ in American terms so the stated warmth rating seems to be roughly correct.
