Here's a beautiful mottled Pumpkin Lapp Multipart I picked up in a small collection recently. I took a photo of the incuse marking which helps id its age. Brent Burger quotes from Jack Tod's "Porcelain Insulator Guidebook" that the incuse marked pieces indicate the production was between 1919-27. (Thanks B!)

Paul Greaves looked at the diagram in the book to see what date matches the upper right notch (2 o'clock) on the oval. He says this dates it to 1925.

The top shell is 9" wide and the bottom shell is 7" wide. There are "inner skirts on both upper and lower shells with a diameter of 3.8" upper and 3 inches lower. The entire unit is 5 1/4 inches tall. The insulator is now identified as an M-2605.

For a close-up of the Lapp Insulator logo [id=185394485] Thanks to B & P for the fast answers!