Three Pence Queen With Interesting Overprint. What Is It?

Any info on this would be appreciated. Looks to have a London cancel. Any info on the plate number of this one? Not that it matters much but the little black dot towards top of stamp is from my camera. Thanks!

The plate number is under the second N of LONDON...looks like 21 to me.

[eta: I should have added, it is 1883, #94 in Scott]

Thank you Cjd. Wow, I didn't realize how bag that picture I scanned was! Takes up a big amount of space on the post! lol

You can improve your stamp by straightening the bent perfs which are very apparent in the scan of the back.

The overprint is a surcharge... lots of experimentation was going on at this time and, as stamps were required that could be recognized even under poor lighting,several stamps had this surcharge in carmine. There is only one plate number (21) for this three pence stamp with the "3d" surcharge.
There is a companion stamp in the sixpence value which also has a "6d" carmine surchage.

Thank you Scotz! Very informative. I'm really enjoying learning about the different surcharges and cancels.