Clausager says 'lashing brackets' were added to the front bumper on export MGBs at chassis number 107703 in November 66, and at the same time the towing eye (fitted from car 368 in July 62) was deleted from the front crossmember.

Despite the statement about 'export cars' Bee came to me with them fitted and they seem very common on home-market cars:

Attached using the overrider bolt and another at right-angles through the bumper spring:

The only 'official' reference I have found to these brackets anywhere is in the Workshop Manual page for the 'Sabrina' overriders used on 1974-model CB cars for North America, item 16:

MGCs had a different arrangement of a flat plate that extended through the valance to the front bolt that mounted the spring to the chassis rail, as well as the one straight through the spring as above. The hole for the chain or strap points across the car, as opposed to half-and-half as for the MGB. Various suppliers show this but only a couple give the part number as AHC40, described as a 'towing eye' which is not unreasonable. This type can be seen on early MGBs without front overriders - although in theory the later type could be attached using the chrome bumper bolts - front overriders only becoming standard in August 66. Series production of the MGC didn't start until December 67, so maybe MGBs had the flat type originally, changing to the curved at some point - presumably prior to the 1973 model year at least. Clausager writes that in June 1969 "Front towing eyes (plural) discontinued, replaced by lashing brackets", at MGC car number 8333. Was this when the MGB changed as well? Seems odd the two would be different. Clausager lists no other changes for the remainder of CB production: (Motaclan/Leacy)

Clausager says that lashing brackets were added to the rear of export cars at chassis number 107465 in November 66. Perhaps unsurprisingly Bee doesn't have them - although she does have the front ones, maybe they were used when being transported to dealers. These next two pictures of the brackets were posted on the MG Enthusiasts forum. Some apparently flat: (Dave O'Neil)

And others curved: (William Revit)

Shown quite well in this picture, which also shows the North American half-bumpers for 1970: (Historic Motor Sports)

There has been debate about just how they were fitted i.e. whether they went on the chassis rail bracket before or after the bumper spring, with the tab resting on top of the spring. This again from the Workshop Manual on 'Sabrina' overriders shows flat ones fitted to the chassis brackets before the bumper springs:

However the curved ones are such that it must have been after, with the tab resting on the back of the spring the lower part pointing backwards, and in any case if they went on first it would space the bumper back on cars with them compared to cars without. Also if they were intended to be fitted and removed for shipping it would be much easier to remove just the nut and washer to get the lashing eye off, than have to remove the whole bumper. As to why they were curved, the flat ones are visible in photos of the rear of the car, maybe the curve was just enough to tuck them up out of sight unless you squatted down. Renders them unsuitable as a towing eye though, which the flat ones might be able to do.

Rubber bumper: Vee has them both front and rear, attached direct to the bumper armatures as you can see, although again they aren't shown in the Parts Catalogue or the main suppliers websites: