It won't fit unless you do some modifications.
Pictures would help, what are the pickup routings on the 51?
If I'm not wrong they are H-H, if so, it still won't fit unless you get someone to custom cut your pickguard.
51 pickguards are difficult to come by.
and I've never seen anyone selling HH combi 51 pickguards anywhere.
It can be done, but you need money.