It's hard to tell if this is the right one, but it looks close.

http://www.ebay.com/itm/Datsun-Nissan-Gas-Cap-w-keys-old-style-Genuine-Great-Classic-Datsun-models-/291790584662?vxp=mtr&hash=item43f012fb56

 Thanks SteveJ. I'll pass the info to RedBird. She's still looking for one.

20 minutes ago, Mark Maras said:

 Thanks SteveJ. I'll pass the info to RedBird. She's still looking for one.

Someone who has actually seen the locking gas cap, such as @Montezuma, should probably chime in first to say whether or not that is the right one.

I also found this one: http://www.ebay.com/itm/Datsun-240-280Z-locking-gas-cap-/112011450002?hash=item1a14671692:g:hEYAAOSwOtBXS68n&vxp=mtr

Maybe Jai could write to the seller of this one and ask for a picture/measurement of the underside: http://www.ebay.com/itm/DATSUN-1200-B210-210-LOCKING-CHROME-GAS-FUEL-CAP-NEW-OLD-STOCK-/142005401286?hash=item21102e76c6:g:FykAAOSwa81XRxMQ&vxp=mtr

Another one: http://www.ebay.com/itm/DATSUN-Z-SERIES-1970-1976-240z-260z-280z-Deluxe-A-HEAVY-DUTY-LOCKING-GAS-CAP-/112025793011?hash=item1a1541f1f3:g:bX4AAOSwnFZXXbih&vxp=mtr

 

Excellent. More options. I sent a message to RB. In lieu of a locking cap, she's leaving the cap a bit loose to detect tampering. The logic being, anyone tampering with it would put the cap back on tight. She is also looking at trail cams. 

A motion light mounted to a wooden stob close to the Z, wired into an extension cord?  Or my favorite,

1 hour ago, siteunseen said:

A motion light mounted to a wooden stob close to the Z, wired into an extension cord?  Or my favorite,

 Oh yeah. Great deterrent. Facing a dog (or person) like this you have to ask yourself, how crazy is this SOB. Deanna, RB's dog, is a lover not a fighter. Barking is probably the extreme limit of her guarding abilities.

Deanna's like my new dog, he'd help drag the gas jugs back to the thieves vehicle.  8^)

 

1 hour ago, SteveJ said:

I also found this one: http://www.ebay.com/itm/Datsun-240-280Z-locking-gas-cap-/112011450002?hash=item1a14671692:g:hEYAAOSwOtBXS68n&vxp=mtr

Maybe Jai could write to the seller of this one and ask for a picture/measurement of the underside: http://www.ebay.com/itm/DATSUN-1200-B210-210-LOCKING-CHROME-GAS-FUEL-CAP-NEW-OLD-STOCK-/142005401286?hash=item21102e76c6:g:FykAAOSwa81XRxMQ&vxp=mtr

Another one: http://www.ebay.com/itm/DATSUN-Z-SERIES-1970-1976-240z-260z-280z-Deluxe-A-HEAVY-DUTY-LOCKING-GAS-CAP-/112025793011?hash=item1a1541f1f3:g:bX4AAOSwnFZXXbih&vxp=mtr

 

Thank you Steve.  The last one is really too expensive for me.  The 2nd one, I talked to on the phone and we exchanged measurements.  No go, the inside, filler hole size is 2 1/2 inches.  RB'S are 2 3/5 inches.  So no go.  

The first one I have sent the measurements needed, because he states no refund.  That would be the pits, spend $, have locking cap, and lose on both scores.  So we shall see what he says.  Thanks again!

I bought one of those BMW locking gas caps for my 76 Z.  It is the "larger" style for the large filler neck.  It has the BMW logo stamped into it.  

However, it does not seem to lock the cap on.

I mean, the key will work, and actuate the tab, but I am still able to remove it while "locked" pretty easily...

I still use it though, it fits good and looks better than the old cap.  It does not have a spot to attach the "hanging chain" though...

FYI

Not mine, but this is pretty much exactly the cap I have:

I got one too. Ramsesosirus is correct.

2 hours ago, Stanley said:

I got one too. Ramsesosirus is correct.

I suppose I should clarify that the BMW cap isn't just "pretty easy" to remove when locked, it just unscrews as normally.  So mine is always "locked"...  I spent a decent amount on it too..