So i am trying to secure a centre casement to a removable mullion that makes up a fire escape, i am trying to keep the fixing either out of sight or very minimal. I have come up with somthing that i will mention but other folk may have others worth mentioning.

here is the the removable mullion held in with a secure bolt

1st idea useing sash fastner but its just to big, i know you can get narrower fastners to work would need to be 50% all round smaller

Second idea to add a small plant rebated & fixed into the mullion then passing a 6mm threaded brass pin through it into the casement screwing the casement too, shown a wood block, pin & threaded insert.
Ebay have these https://www.ebay.co.uk/itm/286492707852?var=588577166498
Above pic shows a pin that i can insert through the mullion and have somthing like a push button latch that has a 20mm bolt throw there or there abouts into the casement, this is my preferred method however i cannot find anything on the web so asking you guys have you any ideas or other options worth considering.
Just for info i would be happy simply fixing the casement to the mullion and open them up as one but the swing angle will not allow the mullion out of the frames lower rail.

here is the the removable mullion held in with a secure bolt

1st idea useing sash fastner but its just to big, i know you can get narrower fastners to work would need to be 50% all round smaller

Second idea to add a small plant rebated & fixed into the mullion then passing a 6mm threaded brass pin through it into the casement screwing the casement too, shown a wood block, pin & threaded insert.
Ebay have these https://www.ebay.co.uk/itm/286492707852?var=588577166498
Above pic shows a pin that i can insert through the mullion and have somthing like a push button latch that has a 20mm bolt throw there or there abouts into the casement, this is my preferred method however i cannot find anything on the web so asking you guys have you any ideas or other options worth considering.Just for info i would be happy simply fixing the casement to the mullion and open them up as one but the swing angle will not allow the mullion out of the frames lower rail.
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