Writable gadget does not have an implemented GetAllowable method.

[laurenrad]

Attempting to access the allowable property of the Writable gadget results in an error, as the Toolbox does not recognize this method. This looks like another one that was going to be implemented at some point that wasn't. Maybe this accessor could be left in place but just return None?

[riscman]

I was looking at this today and was going to report it but you beat me to it.
It is plain from the Toolbox user interface documentation that it isn't supported.
SetAllowable 514, followed by SetFont 516. It is even clearer in the Strong Help manual:-
513 - WritableField_GetValue
514 - WritableField_SetAllowable
516 - WritableField_SetFont
I was a bit confused by the numbers at first because you use hex in your code and the documentation uses decimal. Hence &203 = 515

[c-jo]

I wonder if this is something that got fixed for Select, but never made it over to the RISC OS 5 versions.