[ricm]

OK...

Over the years Tamiya have released different versions of the coupler:

1) basic coupler, for use without MSL or MFU (released with early tractor truck kits such as King Hauler and Globe Liner)

2) coupler for MSL (released with the MSL, to swap out with the basic coupler above - hence your reference dates go way back)

3) coupler for MSL and MFU (originally released with the MFU kit, now comes as standard in some of the newer tractor truck kits)

I've looked through the Knight Hauler instructions.....the 5th wheel as supplied as standard in the Knight Hauler kit is type 3), and therefore compatible with both the MFU coupler switch AND the MSL. You can ignore the 5th wheel provided in with the MSL....you are correct that the 5th wheel in the MSL does not make provision for the coupler switch so DON'T USE IT! You can tell the designs of the 5th wheel apart because of the shape - the one you want is oval in shape, the one with the MSL is round.

Step 35 (page 19) in the manual shows how to assemble the 5th wheel, as provided with the basic Knight Hauler, with the coupler switch (shown as optional extra).

Then BEFORE you assemble the 5th wheel to the chassis, go to page 26. The bottom of the page shows you the extra/ alternative parts you need to assemble (taken from the MSL kit) to then make your 5th wheel also work with MSL. You can see that you need to swap out a few parts you have assembled in step 35 (a ball joint BE5 and nut BD3) for alternative parts from MSL, so you can add the sliding bar. The assembly of the same parts is shown a little more clearly in the MSL instructions, make sure you look closely to check you have the right washers, nuts, etc. from MSL for the sliding bar to work properly. Note that in addition to building the 5th wheel correctly, you need to use the plastic plate (that goes between the 5th wheel and the chassis) from MSL kit, NOT the one shown as standard assembly (part G8 in step 35).

My advice for the bar again is don't tighten things too much - the bar needs to easily slide WITHOUT activating the coupler until it reaches the extreme of movement in one direction. So the coupler should easily close under the force of the spring without moving the bar, if the bar is in a middle position. You'll get the idea when you put it together..

Hope this helps!