The specific model number for the second Seiko produced divers strap has previously not been known in the community. Thanks to the appearance of a couple of examples in Japanese auctions over recent months it seems that the official model number has been identified.
This model appears on the rear of the packaging for examples of the replacement Chocolate Bar a.k.a. Captain Willard strap. While this may be an updated model number from the initial release in 1969, as it is a spare part replacement strap, it is an official strap number from Seiko.
The strap number can clearly be seen on the stamp from the back of the packaging and it also shows the original watch model (6159-700) that the strap was provided on.
This strap was identified by Luka (@lilseikos on Instagram) from a recent Japanese auction, from seller y_wakako0414, and is the second occurrence of this spare part to be seen offered for sale. My thanks to him for the assistance identifying this.
I have updated the main article relating to the strap to note this new information - Second Seiko Dive Strap - GL721 Chocolate Bar. I have also updated the general summary article on the Vintage Seiko Dive Straps.