Here's something I found through Google:

Originally Posted by: some Rickenbacker FAQ siteSubject: 5.11 What are the types of magnets used in the different kinds
of pickups?

"The vintage reissue guitar pickups use cylindrical Alnico magnets... like
the originals. The vintage reissue bass pickups use tungsten steel
'horseshoes' as polepieces... like the originals ... coupled to ceramic
magnets (instead of the tungsten being magnetized like the originals). Our
humbucking pickups use Samarium-Cobalt magnets. The modern guitar and bass
pickups use "rubber" magnets . . . which are actually a zillion magnetic
bits supported in a synthetic block.

Each has unique properties for which they were selected to optimize
performance in a given application."

[John Hall, [email]ceo@rickenbacker.com[/email], 5/28/1998]

"The principal difference is that the High gains have a single strong
ceramic magnet on bottom that contacts 6 screws that are capped with
buttons and the vintage reissue toasters have individual alnico pole
piece/cylinders under each string in the bobbin wound with magnet
wire--almost exactly the same as the original old ones(just the corners
of the new bobbins inside are a different shape more 4 corners looking
whereas the old ones were 1/2 round on each end ---but you wouldn't
know if you did not take one apart.

They do sound very much the same -- the high gains and the toasters of
today, just slightly different!! The high gains being louder as you say.
This is not so however when you compare 1950's & 1960's toasters to
today's high gain pickups and reissue toaster pickups.---go figure??

The original toasters were brighter, clearer, and more HiFI/lower
distortion and lower output compared to today's-----most of the middle
60's toaster pickup coils were 7,500-8,000 Ohms ---- the reissue
toasters are 11,200 - 12,700 Ohms or thereabouts which makes them sound
more like the standard high gains than like the original toasters.
Late 50's and early 60's toasters were even brighter and lower output
than the middle 60's toasters were usually being somewhere around
5,000 Ohms.

The beauty of the reissue toaster is that the physical structure is the
same and if you like, you can easily recreate/cause it to sound like
any real 50's or 60's toaster pickups sound/makeup by simply having
the coil/bobbin rewound to the same value with the same wire as the
original you like and get substantially the same sound as the original
vintage non reissue pickup!!