I picked up several of these at a garage/estate sale. They have a 20 rd capacity.

Any idea of their worth?

Are they made by Sterling in England? Unless that, they probably arent worth much more than normal 20 rounders.

I may want to buy them if you are thinking of selling

Curved 20 round AR-18 mags?.. Piccie please. (Probably USA)

Meplat-

Stemcell you just might have two 20rd parkerized steel Sterling-England mags if they they fit the description.  Floorplates should be marked "Sterling-England".  Not as much demand for the 20rd parkerized steel Sterling mags compared to the 30 and 40rd so I would price around $50 each.

Kobren- I owned an AR-18, and still have the mags. two are Sterlings. They're straight, and aluminimum. These sound like "Other" mags.

Meplat-

These are marked "Sterling", and are parkarized.
They are not "other" mags.

I have a feeling they are rare because I've have only seen the straight, aluminium ones.

This can be solved easily. Post a picture. Both a side shot, and a floorplate shot.  Either they're Sterling 30 round mags, or someone slapped Sterling floorplates on cheap 20's.

Again, I have never seen a curved 20 round mag, of a reputable manufacturer, for the AR-18.

Meplat-

My dad has a Sterling, a Howa, and a Costs Mesa, all in mint condition. He needed some extra mags for them, so he inquired of the factory about accessories and received a letter of what was available. He probably still has the original letter, since he hates to throw anything away.

On this list were different mags(20rd, 30rd). The 20's were offered in both aluminum and steel. Since he already had some Al ones, he ordered the steel ones and got what you described.

I don't think a repudable company like Armalite is going to take someone else's mags and put their base plates on them.

Judging from the feedback you are getting on this, no one seems to know much about them, so I'd say they are RARE.

I'll ask my dad if he'd like to sell any of them. If so I'll IM you.

Hope this helps.

If my computer had the necessary accessories to do that I would, But it doesn't.

I have in my collection at least one sample of every ArmaLite factory mag that was ever mfg for the AR18/180.  The mag in question is the curved 20rd parkerized steel mag marked on the floorplate "BY STERLING-ENGLAND" and the mag has slots for both the AR18 and AR15.  This is a legit factory mag that was mfg by Sterling England for the select fire Sterling AR18 but also was sold by ArmaLite as a accessory to the Sterling AR180.  The Sterling curved parkerized steel mags came in 20,30 and 40 round capacity.  The Sterling AR180 was sold with a standard straight black alloy 20rd mag marked "AR18 ArmaLite Inc." which was mfg in the USA to the best of my knowledge.  I have numerous ArmaLite AR18/180 factory brochures, manuals and vintage magazine articles for documentation of this info.  Hope this helps, Kobren.

Thanks, but getting back to my original question,
"What's it worth?"

I estimate that they should "sell" for $45 to $65 depending on condition.  Kobren.