I am driving a 1987 300E. Recently purchased a distributor cap from Fastlane, but it looks nothing like the part that is installed.

How can I tell if this engine is actually a 260? The block casting has the number M103 011 04 01 on it.

Also, the wire loom is a lightweight plastic box, maybe 20"x2"x1/2" that sits in a recess that runs along the top of the valve cover. It's pretty brittle after 20 years on a hot engine. What's the best replacement path?

If you have a 1987 300E then it has the 103.983 engine (3 Liter). The 300E with 2.6 L motor was produced in 1990.
You probably have the correct distributor cap (FYI-both the 3L and 2.6L engines use the same cap)
The ignition wire holder costs about $12 and can be purchased from Fastlane (click on "Buy Parts" tab)

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

one further question, por favor. I have the shop manual CDs. How do I find info on changing out the rotor and cap without moving to a monastery for several weeks of quiet contemplation?

http://www.peachparts.com/Wikka/M119DistCapRotor

Remove the bottom bolt first (5mm ALLEN). This is the most difficult one but it's a snap with a little tip and the right tool. The plastic fan is in the way. But the fan blades are not evenly spaced. Spin the fan around and you'll notice a spot where the blades are much farther apart, this is your window to the bottom bolt.

Use the allen wrench tool described earlier and attach it to a ratchet. A 1/4" drive ratchet works best but a 3/8" drive ratchet and a 1/4" adapter also work. Use the top of the belt as a guide to find the bolt. Once you get this bolt off remove the other two allen bolts on the top left and top right of the distributor cap. Wiggle the fan and play around with the distributor cap to free it up.

__________________
'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

may you live to be a thousand years old, and people the earth with your descendants.