OneGuyFromBarlick

You are an engineer, so is it some sort of thickness gauge, a reference thickness piece? Sorry, don't know what the technical term would be, but they come in sets of different thickness.

Calibration piece or test piece? Would that be the correct term?

Every time I look at the rebate in the side, I think of Glass, is this something to do with sliding glass doors. Also noted the top appears to be pinned,could be a plunger.

A weight, for checking scales.

All ingenious but nowhere near yet. Nothing at all to do with engineering. Big clue for you, if you needed this you'd carry it in your pocket.

Does it hold a liquid Stanley??

I still think it has a U shape, so is it for holding notes? or holding a bob of matches? Grabbing at straws a bit here.

I actually thought about matches, Gloria.

The frustrating thing is that the object is somehow so darn familiar. I have seen these before, but have been putting the old brain through hell trying to recall where and when...and what it is. Turning myself inside out. ( I HAVE seen them!)

Not a plumb bob by any chance ( I am telling myself NO even as I type).
Stanley, if I don't sleep well tonight it is your fault...

Is it for starting fires,like a fire steel.

Everlasting match?

You were all skirting round it but the last one got it. The groove down the outside is for striking the spark on the steel contained in the match and Cath was right in that it contains petrol. Don't ever bother getting one, they are totally useless (or at least, this one was!). Try this one.....

Is it a shepherds whistle.

Nope. It doesn't make a sound.

Bottle top remover ?

I see what you mean but no cigar!

( cruel...you know I have given up smoking... Though I have never smoked a cigar)

Could it unlock a window security lock? I am wondering why it is on a string. Obviously so it can be kept handy. What sort of things need to be kept handy? Things you use often, carry with you, or need in an emergency. What sort of things do I keep on strings or ribbons...keys, window locks

Are we looking at the back of a watch---the upside down type nurses wear? or a compass?

Having said that, the string is neither dirty or worn, which suggests it is not an old or well worn object. The string looks a bit like a fine Venetian blind cord, but that could just be your nature to use ' a bit of anything you have for the job'.
Could it be something to do with your new heater?
( why have it on a string? I t must be needed on hand ' at source of need', or need to be carried with you, or easily lost if not in a certain spot).

Can we see it from the other side, or would that give the game away?

Sitting here next to Mr Feeby watching My Kitchen Rules. I told him I did not get the cigar. He looked at the pic and said it was a CIGAR CUTTER!
No...!...really?!
Are you kidding me?

Its a sos Talisman necklace pendant..

Rotating stepped ring puzzling me, does it have anything to do with measuring miles on a map.

I'm impressed by the way you were all working towards it but Young Tom has got it. It's a universally recognised device that holds a small piece of thin paper with vital information on it which goes beyond identity. It has things like allergy information and whether you have made an advance directive about treatment. They can be either pendants or bracelets. Maz was right, it's braided nylon cord. I am on the verge of investing in a gold or stainless steel chain.
I'll have to do a bit of thinking now.....

Made by Robert Lingard of Sheffield.....

Thread nipper/cloth trimmer thingamajig?

You're getting there but a very specific use and user. More information. (Comrade Nolic will know this one!)