I'm a little confused...do you have that backwards?
Clipping and Pinning are modifications to allow you to stay within the laws/regulations. How can a c/p'd lower without a registered sear be considered an illegal machine gun device?
As far as an Sp89/Mp5k conversion... How does one legally use a 4position mp5k lower? Acquire a 4position clipped and pinned lower and swap in the Sp89 internals along with an mp5k selector/axle? Am I thinking along the correct lines here?
Actually, clipping and pinning are not done "to allow you stay within the laws." Full-auto packs won't fit onto the semi-auto receivers because the shelf on the receiver is wider than the space between the ears on the front of the lower, where the swing-down pushpin goes, and also, the internal pack frame has a corner that also would interfere with the semi-auto shelf. Clipping and pinning is the act of machining out the inside of the ears and enlarging the hole in the front of the grip frame so it fits over the shelf. That's the clipping part. Pinning is taking a pushpin and cutting the ends off, and then super-gluing them into the grip frame to make it LOOK like there's a pushpin there. Then to make a burst or SEF pack fit, you must also cut out the lower corner of the pack frame so it clears the shelf as well.
And it's not backwards, but I left a phrase out, so I can see why it's a little confusing. When I said "without a registered sear", I should have written "with a factory sear", because that's the way they're usually sold - complete burst packs with the catch and trip lever. In fact, the first used burst pack I bought had already been clipped and pinned. Yes, it was an illegal conversion device. It would fit on the gun, and by holding the trigger down and manually cycling the cocking lever, it would fire in semi, burst, and full auto mode. It was illegal as hell, so it and the gun were never in the same location at the same time until after the pack had been converted to take my registered sear, and even then, that pack was technically a machine gun, all by itself.
To use a registered sear in a burst pack, a few key modifications must be made because registered sears and factory sears (actually called "catches") are not the same dimensions. The key mods are that the catch pivot hole must be moved slightly, and the catch notch in the hammer must be extended.
So... if you were to find a clipped and pinned lower that is otherwise unmodified, even if the catch is missing, you still have an illegal conversion device, because the factory roller catch is not a controlled NFA item. In the eyes of the ATF, since all you have to do to make it operate in full auto mode is reinstall the factory sear, what you have is a machine gun.
Furthermore - they recently ruled that even packs that are clipped and pinned and have been modified to accept only registered sears or "sear ready" are illegal if you do not have a registered sear. I don't think they enforce it quite like they would a clipped and pinned fully factory pack that is 100% operational, but they have made the ruling.
The only way to stay 100% legal is to get a completely unmodified pack, and do not clip and pin it until you either install a registered sear, OR modify it so it cannot possibly fire in burst or full, but if that's all you want to do, there's no point in getting a $500 burst pack. You just need to put an ambi-semiauto pack into a burst housing.