Here’s an old link that discusses the King Seeley in more detail.
https://www.fordbarn.com/forum/showt...eely&showall=1
These guys possibly do repair, but aren’t taking new work.
https://calibratedclassics.com/servi...ender-services
I believe you’re correct about the wagon having a unique sender. I think the tbird guys still sell replacement gauges, you may be able to use one of those to rebuild yours.
Does the wagon have access to the sender thru an inspection plate like the cars and birds do?