My understanding is that of the real 5" clocks there were the two variations you mention, where the right angle box either clamped to the nipple, or was held down by a horseshoe shaped plate screwed to the case. Both of these apparently lacked the threaded nipple to accept a cable directly, and had to be used with the appropriate angle drive.
However, it was apparently a common mod (done by Auto Tempo among others) to modify the speedo so it had the standard threaded nipple which could accept a cable directly in order to do away with the troublesome angle drive. This considerably improved reliability.
Speedos with the straight-cable mod could also be fitted with the later style Smiths BG2410 angle drive which had a round knurled nut that screwed down onto the nipple and could be used on almost any speedometer that accepted a standard cable. This is the gearbox used on most of the repros. Since having the cable come straight out of the back looked like hell, some original speedos so modified doubtlessly ended up with the newer style drive as owners tried to revert to stock appearance. And some rebuilders apparently used the later knurled-nut style boxes when repairing original 5" clocks once the original style boxes became unavailable, back in the days when riders were more concerned with function than originality.
The Smiths BG 2410/00A knurled-nut gearboxes are no longer available (though they turn up on eBay quite cheaply) but an even clunkier version is still available from Land Rover dealers.
There remains some question as to whether 5" speedos were ever supplied by the factory with knurled-nut style gearboxes. My suspicion is that they were not and that, apart from a few modified original 5" clocks, just about all the 5" clocks with the big gearbox are later replicas.
BTW someone was selling a "genuine, original, NOS Smiths 5" speedo" on ebay a couple weeks ago, which appeared indentical in every respect including the late-style gearbox to my Auto Tempo. It sold for $1550.
There were two designs of angle drive on the back of Shadow speedos. The first type was in brass (I think) and secured with a little clamping band that nipped up on a projection of the mechanism that protruded through the back of the case. The second type is in Marzac secured by a plate with a "U" shaped cutaway which slotted in groove in the angle drive. the plate was fastened to the instrument with two 4BA screws. This type can be dismantled for cleaning and luubricating by removing the two small internal circlips, something you need to do to avoid broken cables,