Trevor, in early 2012 I had Byrd heads made for a KP312 and another large beam planer. I dealt with Holbren Sales, the salesman's name was Dan, I believe. So Holbren and Byrd ought to have the specs on file for the KP312. (Rather than take the measurements--and liability--myself, I removed and sent the head to Holbren to be measured.)
We work almost exclusively with hardwoods, and a good bit of our material is to some degree air-dried. So these heads have been very helpful to us. I found the edge quality of Makita's blades to be excellent. Indeed, surface finish from Makita's blades when new is downright silky compared to the results from the Byrd. But after several hundred feet of use the Byrd becomes the winner, and continues to plane without tear-out in sloped grain for a long time.
Another aspect of the Byrd head... reduced noise.
One caution... the cutters are of course very brittle. And absentmindedly sliding a portable tool like the KP312 along, say, the shelf where it's stored, can, in a moment, lead to 50 bucks worth of broken cutters. Or $100, if one is a slow learner like myself.
