'should' used as 'if'

[White Hat]

The use of such leaflets could have been a lot more effective should they have been used between 1944 and 1945.

Is 'should' used correctly in this sentence?

 

[White Hat]

The use of such leaflets could have been a lot more effective had they been used between 1944 and 1945.

Is this a better version?

 

[5jj]

The use of such leaflets could have been a lot more effective should they have been used between 1944 and 1945.

Is 'should' used correctly in this sentence?

It sounds unnatural to me

The use of such leaflets could have been a lot more effective had they been used between 1944 and 1945.

Is this a better version?

Much.