16h

Insufficient space between tyre and guard, to get a tool onto a nut or bolt head? Maybe you need to bend a spanner.

Do you have 3.50 instead of 3.25 rear tyre?

Fixing on my 35 duck tail were through to captive nuts so no spanner required.  Maybe it was modified in the past for same reason.

if the bent spanner solution does not work let the tyre down, zip tie or clamp it to the rim to give you clearance in the right location.  Then blow it up again. 

J

Hi Beverley,    There is normally a metal strap with 2 captive nuts attached. This fits under the main mudguard and when the tail-piece is fitted it clamps the 2 parts together. When you want to remove the tail-piece you just need to loosen the bolts and slide it from the slots in the main guard. You could make a new strap from a 1/4" x 5/8" metal strip bent to shape with 2 threaded holes which will be thick enough with-out nuts.

This is the fixing strip on the post-1937 rear guard (with double top number plate bracket fixing)

Some tyres are fitted in reverse to the arrows for good reason . A tyre designated as front only are often  fitted in reverse to a rear wheel to accomodate the way the construction works with thrust vector . Blimey! that sounds like I might not be clueless after all  !!.

Some people might buff the arrows off.

4.10 is very big.  Books list 3.25.