Normally you find on the bolt handle the complete number (up to < the last >four digits).(Same as action number)

If there are clearly visible only two numbers on the bolt handle they would have chosen the two last numbers of the action(In your case 75 ).

That number (75) is normally found on the floorplate(inside !) and the small parts (striker ,strikernut,boltshroud,safety/if not removed and showing safe-fire.

On the backside of the magazine box 4 digits.

These numbers were all part of the Oberndorf serial number.

Rigbys mostly added their number (4 digits) to the Oberndorf numbers on the receiver ring and the triggerguard bow.

Sometimes you´ll find only the Rigby number on the receiver ring (and the Oberndorf number on the bottom of the action or left side of the receiver ring under the wood line.

So you should have a look at all numbers and than you can decide if all those numbers match.

So the only explanation for your item if matching numbered could be that by chance the Oberndorf number ended with 17 .

Reads somewhat complicated , but hope it could help you.