Technical blocks/Skull

Java Edition
1.4.2	12w36a	Heads have an extra, unobtainable item form corresponding to its block ID. It can be obtained via the `/give` command or inventory editors with numeric item ID 144.
		Unlike the proper head items at the time, it had a placement sound, and was capable of directly replacing replaceable blocks. (The proper head items could only replace them indirectly, i.e. they could be placed against the side of a block which would result in it replacing snow, however could not replace snow if the snow itself was targeted.)
		The head item, if placed by a player facing north, east, south or west would result in the placement of a skull block with metadata 0, 1, 2 and 3, which were, in order, an east-facing skull visually wall attached but with a floor hitbox, a north-facing floor skull, a north-facing wall skull and a south-facing wall skull.
1.7.2	13w37a	The direct item form of heads has been removed from the game. It can no longer exist as an item in any way, only as a placed block.
Pocket Edition Alpha
v0.12.1	build 1	Skulls exist as an item.
		When a mob head is mined using a tool enchanted with Silk Touch, it drops both its regular item form and the skull block item.
		The block item places creeper heads that have no collision box and are attached to the side of the block where the head faces away from the player. When the player is facing north, the placed creeper head faces east-northeast instead of north, which is a normally unobtainable block state/NBT combination. Block placement only succeeds if there is a block below where the creeper head is being placed.
	build 10	The skull block item now places invisible mob heads instead of creeper heads. Both newly placed heads and previously placed creeper heads now have a collision box.
v0.13.0	build 1	Skull block items no longer drop from mob heads mined with a Silk Touch tool.

Appearances

Java Edition
1.4.2	12w36a	The skull item uses this texture in inventories, when held in first or third person view, as a dropped item or when in an item frame.
1.5	13w02a	Due to major texture storage changes in this version, the skull item no longer has a valid item texture, making it cause a game crash when held in first or third person view, as a dropped item or when in an item frame.
1.5	13w02b	The skull item now uses this texture in inventories, when held in first or third person view, as a dropped item or when in an item frame.
Pocket Edition Alpha
v0.12.1	build 1	The skull item uses this texture in inventories, when held in first or third person view, or as a dropped item.
Bedrock Edition
1.10.0	beta 1.10.0.3	Skull items now use this model.