A fishplate template

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I'm definitely no PW expert, I've been looking at this for ages and still can't figure it out.  Is the inside section meant to be the cross section of a fishplate, but why the curvature ,and what are the other offshoots relating to?

https://www.easyliveauction.com/catalogue/lot/0b8b5b346ee9794d287f2f2b15e2f283/0af8d24542e81eb9357e7ef448a6646f/general-railwayana-online-sale-650-lots-all-small-p-lot-407/

[jhb171achill]

????

 

Wow - no idea!