The Lahd (a side extension of a grave) and the (straight) cut in the grave

Narrated Jabir bin 'Abdullah:

The Prophet collected every two martyrs of Uhud (in one grave) and then he would ask, "Which of them knew the Quran more?" And if one of them was pointed out for him as having more knowledge, he would put him first in the Lahd. The Prophet ﷺ said, "I will be a witness on these on the Day of Resurrection." Then he ordered them to be buried with their blood on their bodies and he did not have them washed.

حَدَّثَنَا عَبْدَانُ، أَخْبَرَنَا عَبْدُ اللَّهِ، أَخْبَرَنَا اللَّيْثُ بْنُ سَعْدٍ، قَالَ حَدَّثَنِي ابْنُ شِهَابٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ كَعْبِ بْنِ مَالِكٍ، عَنْ جَابِرِ بْنِ عَبْدِ اللَّهِ ـ رضى الله عنهما ـ قَالَ كَانَ النَّبِيُّ ﷺ يَجْمَعُ بَيْنَ رَجُلَيْنِ مِنْ قَتْلَى أُحُدٍ ثُمَّ يَقُولُ " أَيُّهُمْ أَكْثَرُ أَخْذًا لِلْقُرْآنِ ". فَإِذَا أُشِيرَ لَهُ إِلَى أَحَدِهِمَا قَدَّمَهُ فِي اللَّحْدِ فَقَالَ " أَنَا شَهِيدٌ عَلَى هَؤُلاَءِ يَوْمَ الْقِيَامَةِ ". فَأَمَرَ بِدَفْنِهِمْ بِدِمَائِهِمْ وَلَمْ يُغَسِّلْهُمْ.