HI,
The calibre you need to put down is .45/577, .45 is the calibre of the rifle.
The case is .577.
Type :Rifle.
Action:Single shot.
Id No. as stated-no visible number.
Because the rifle is classed as an obsolete calibre it is technically an antique and you can take it off your FAC when ever you want, and put it back at a later date.
You must remember to inform the Firearms Dept of any changes, and they get a bit miffed if you keep putting it on and taking it off.
When it is on your FAC you need to lock the gun in a cabinet.
If you take it off your FAC you can hang it on the wall.
Hope this helps.