How to lift a coal hole cover plate – a dilemma

I have often pulled together collections of coal hole cover plates showing how diverse the range of designs can be. Most of them sport a pattern on them to make them anti-slip and often used as a means of promotion for the hardware store or iron founder. They show no visible signs of how the covers/lids might be lifted and evidence about this process, or diagrams to explain the construction, has proved elusive or inconclusive*. 

Whilst presenting a talk about coal holes at London Historians History in the Pub evening last month I asked the audience for ideas about their usage and the consensus was that the scullery maid would have pushed up the cover plate from the coal bunker below so that the delivery man could then remove the lid by hand. He would them drop it back into place after delivering the order. Being flush to the pavement, with no protruding lips to get a grip on, meant the lid was unlikely to be tampered with, or stolen/removed, and the coal below was as good as inaccessible via a short narrow access pipe barely big enough to squeeze your head through.

Some plates were 'self-locking' as shown here. If you look closely you'll see that there are little notches/gaps in the design within the outer circles and it has been suggested that perhaps these needed to be aligned to make the mechanism work. I think this is very dubious idea being as in this example we can see there there are four gaps at 90 degrees other on the cover plate, yet there are six at 60 degrees on the rung. And how would you rotate it?! 

The explanation is that a catch was released from below ground and this enabled the cover plate to pop up proud of the pavement, making it easy for the coal to be dropped down into the hole. The plate was then dropped back into the outer casing and pressed flat to the pavement, causing locking mechanism to click back into position. However, these self-locking plates must have proved unsuccessful seeing as additional locking mechanisms were later added as per here.  

*Hayward's of Union Street, Southwark, were the B&Q of their day and as such would most certainly have produced an extensive catalogue ... do you have any reference of these?

All suggestions and further info is most welcome. Please use the comments section below or email me at jane@janeslondon.com