Bill Webber wrote:
>Needs to be serviceable.  #12 round is the 
>convex iron, about 3/4 inch across.

Having been burned (although unintentionally) in such a transaction,
I'll just that point out there can be quite a bit of variation in the _exact_
size (and curve) for the hollows (or rounds).  I.e. particularly on
older planes the sizes were never completely standardized.

Now, things's will _probably_ be OK.
But, you should consider if you'd be willing to do any necessary
grinding to "fine-fit" the cutter should that be required (but if
the new iron is, say, too narrow, this might not be possible).

If not, you might want to try to negotiate a "return if necessary"
deal with your seller (or decide if you'd be willing to swallow
the cost in the worst case).

(As an aside, if you've got the old cutter, or the partner hollow
one thing I've done is send the seller a photocopy of the cutter
I had so that they could compare it the cutter they're trying to sell.)

One final thing to watch out for is that neither your plane
nor the "source" cutter is from a skew plane (such things do exist;
the "double-harlequin set I bought from a dealer in England
included several).  In this case all bets are off.

Just sayin'
Nichael