Thank's Mr. B.Pete Young's article about Pillion seats-
https://occhiolungo.wordpress.com/2012/08/16/pre-wwi-pillion-seats/
Some first rate ideas there indeed. When I looked at the photo Ralph posted on page 161 of the Indian with the torpedo tank and the pillion I noticed that it appears who ever made the rear seat supports seems to have used bicycle forks to do it.
Most of the pillion seats seem to use flat steel for the supports.
At least the down tubes from the seat to the rear axle look like bike forks and I think that the support from the front seat to the rear seat look like bike forks as well but maybe a different make. Maybe time to check eBay to see what is available for bike forks.
Steve.