RE: F161 Part Question
To utilise the chicken strap option, the stock (100mm) screw should not be too far off the correct length (using stock TT fin or similar insert depth). Most guys discard the front rubber surround safety washer (itself a couple of mm thick) and insert the fin screw into the strap end (internally, under the strap end cover, as per normal deck insert fixing)
using just the standard flat (disk) washer. Overall, once cinched that adds only a couple of mm extra height (required screw length) overall.
Should work fine - although with no instant access to a 161 today that's not finitely tested.
M6 x 110 - 120- 130 mm are not common spares for the wider range of TT boxes, but are in principle supported as spares under the 2007 Hybrid Race and Gemini product programs. In many locations around the world, it's also easy/er to access M6 screws through hardware, marine chandlery etc, however in the US most of the stainless ranges near this size are still based on the US 1/4 x 20TPI system (which won't cheer you up but at least you know).
Dunno what size Trident sent you, but after all this if it remains a problem please advise your details via PM and we'll arrange to send a replacement 110mm to you.
Cheers ~ Ian