New lock
A - shaft that operates the switch and lock bolt; B - keyway for switch location; C - rubber cover for switch; D - Key number; E - Shear-bolts; F - Lock bolt; G - location boss around bolt.
Drill up into the old shear bolt shanks (new lock pictured for clarity). Note that this can only be done with the earlier front-entry locks mounted lower down the column, not the later side entry accessed through the cowl as the bolts are in line with the lock instead of being at right-angles to it.
Parting the multi-way connector with outside circlip pliers, to get the indicator switch harness out of the way
Old lock removed, I'd managed to drill right up the middle of one bolt and only slightly off to one side of the other.
When fitting the upper part of the lock clamp, the hole A seems to be positioned on the key-side of the fixing holes. Whether this is important or not I don't know.
When fitting the switch back into the new lock, having the clamp loose so the lock hangs down (gaps at A) makes it easier to access the switch fixing screw (B). There is a circular hole in the column outer tube (C) and a corresponding boss around the bolt in the lock body, to ensure correct positioning.
John Hall's 'eared' switch on a 1973 MGB in Queensland Australia:
The connections, in the fitted orientation as above, largely from a drawing by Dave O'Neil:
The detail of 'eared' and circular switches, for those so inclined.
There are three part numbers for lock item 10 - BHA5709 (Dec 70 to Feb 72), BHA5215 (Feb 72 to the end of CB) and BMK2259. For the switch item 11 there are two - 37H7708 (with BHA5215) and 37H5934 (with BMK2259), there is not a switch or anything else listed under BHA5709. BMK2259 is described as a service replacement for all previous types of CB locks, and is also shown as 'use prior to BHM7056'. BHM7056 is the rubber bumper side-entry lock under the cowl, so completely different, and the 'use prior ...' statement is not an indication it can be used on RB cars.
Screws could be a clue as to which switch and lock were used when as there are two for the eared switch and only one for the round. But the only screws listed are for the even earlier lock 13H4862 and switch 27H6237 with 2 screws (27H6238) which is a strong implication that was the eared switch and lock, none are listed for BHA5709 or BHA5215, 1 screw (37H6967) is shown for BMK2259 again a strong implication that was the later cylindrical switch and lock, and none for BHM7056. It's tempting to conclude from that the screws remained at '2' until BMK2259 when it changed to '1', implying that locks prior to BMK2259 were eared. But that would be making quite a big assumption, and if nothing else even official period documentation is not immune from mistakes. Also various MGC and early Midget catalogues show locks with two screws 27H6238 (13H4862, which is an early MGB lock) and a 'cap - switch', but the drawings are completely different to any of the MGB locks showing a cylindrical switch, being held in with a circular ring (the 'cap - switch') secured with screws, and the screws look more like the single one for the cylindrical switches rather than the two for the eared switches:
I can't find any pictures of that lock, or description for either screw that would indicate their size, but the implication from that drawing is that they would be completely different to the screws for the eared switches and locks. The MGC catalogue also indicates that BHA5215 is a later replacement for that lock.
The Catalogue states that BMK2259 is a replacement for all the CB steering locks, so it's tempting to think that is the only one the suppliers will stock now, regardless of what part number they list it under ... but they confuse the issue by a factor of 10. They frequently say to use BHA5215 in place of BMK2259 showing round locks and switches. One lists one part on the steering column page and the other on the switches page. Moss says to use the CB switch up to chassis number 410000 which is completely wrong as that is the 1977 model year, and show one switch for both original and replacement locks - but that obviously depends on what you have now. B&G seems to be the only MGB supplier showing both types of switch but only have a price on the eared switch, indicating that to get the round one you have to buy the complete lock assembly. It begs the question of just when that eared switch was used, and what the lock number was.
I was obviously fortunate - again - to get a new lock that was completely compatible with my existing round switch, when the Catalogue indicates (and John's example above of the same model year as mine shows) it should have eared. Maybe that means my lock had been changed before - possible as I did note that the shear bolt heads were missing, when I have it from the purchase of a new car in the 70s that the factory did not shear them when fitting. And even that could have been fobbing me off when I queried it with the dealer.
All the different components, part numbers and dates can only be of academic interest now, and after all this time what came out of the factory could well have been replaced in the past, as is possible in my case. The crucial point for current owners is that for a period of time locks and switches were eared, and at another time they were round, and at another circular but retained in a completely different way. The first two are definitely not compatible with each other, and the third may be incompatible with both, so care is needed if intending to replace only part of the lock/switch assembly.