|I've promised this to Eric (Blakbird and author of Technicopedia) longtime ago, but meanwhile thought it would nicely fit here at TBs too, so it took a long in my post queue...|
This is an addenda to the old 855/955 Mobile Crane instructions, which I found at an eBay auction and won. It is a good example of a set that got some changes by LEGO, while selling on the market. Back to 1978, it was one of the most impressive sets at the time (and somehow still today in my opinion), together with other early TECHNIC sets (Branded Expert Builder Set in US, at the time).
The seller's description mentioned,
"It's a late version of the kit, with the additional parts to make the jib rest which was not on the original version. This is so the arm can be raised easily from the lowest position. The addendum to show this is on a separate sheet, later it was incorporated into the instructions. That makes this kit even rarer."
As Eric described at his Technicopedia 855/955 Mobile Crane technical information page,
"The gear racks are part of a translating rack assembly. A pair of pushrods connect the rack assembly to the boom. As the rack is translated aft, the pushrods drive the boom up. At the beginning of motion, the mechanical advantage is quite poor because the pushrods are almost parallel to the boom. For this reason, a lot of force is required at the crank to get things moving. Once the boom has been raised about 15 degrees, the crank turns easily."
That might be the reason why LEGO later added a boom rest to this model. It would prevent the boom from going all the way down, making it easier to raise the boom by moving the correspondent wheel.
At the time I found also a similar addenda with instructions modification for 853/956 (Car/Auto Chassis), which I wasn't able to find later anymore. It did relate with strengthen steering design, which breaks apart easily into early sets design. Apart that addenda, it is common to find different instructions books for this set, with differences on this matter (maybe a difference between USA and Europe building instructions. Fold-out poster format instructions were also used for Europe in the beginning, instead of more generally used book format).
Several other TECHNIC sets are known to have suffered small changes while on the market (not to mention the re-editions), but usually it resumes to part variations (color and/or design), at least for those I remember.
Eric, naturally you can also feel free to put this image into your 855/955 page at Technicopedia, to complement information about this model, if you wish.