Format of a block key's contents in bitcoind's LevelDB?
This is the Bitcoin Core v0.15.1 source code snippet that defines the serialization of CDiskBlockIndex. The various fields are:


A varint version number of the client who wrote the record (for future extensibility). This is the 87c628 you're seeing.
A varint block height.
A varint status with various flags.
A varint determining the number of transactions in the block.
If the status field has bits 8 or 16 set, a varint determining which file (blk*.dat and rev*.dat) the block is stored in.
If the status field has bit 8 set, a varint determining the offset into the blk*.dat file where the start of the block is.
if the status field has bit 16 set, a varint determining the offset into the rev*.dat file where the start of the block's undo data is.
The block header