Hope you bought a low profile 2.1 and not the earlier standard 2.1 with a higher atty base that was designed for carto's, atty's or modified atty's to go down into their deeper catch cup to reach the 510.
I could make a long list of the original atty's I bought for my Reos...
years ago. But a problem with input from me now days is your quest for a "modern" atty that will fit on it. IOW, I haven't followed any small diameter atty's below 22mm-25mm-30mm for years, and none but mesh atty's for the past couple of years. Your limiting factors will be a diameter small enough to sit on top with no overhang (the Nuppin' is 18mm), their AFC('s) being DL capable or modifiable for it, and being BF ready for squonking.
I first started with MTL BF atty's that I drilled the AFC larger until better/higher quality attys' were found or came out (RM2's, RM3's, Viscious Ant Cyclones. ERA's, etc). I'd suggest a search to see what is available these days in 16mm-18mm BF atty's and then go from there.
Originals I doubt are still available, but you might still be able to get a clone Origen Sweet 16 (16mm) from China.
Thre originals were $100+ (I have 4 of them), but the clones were no more than $10-$15 IIRC. They can be either single or double coil. With both AFC's wide open they are still a restricted DL draw though unless you drill the AFC's larger. They can be a great little flavor atty.
HTH
O-16
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