I recommend you make matters a little easier for yourself.
Stick a little dot on the top of the plug (not collar) after you have initially inserted it in its correct socket (check the collar for orientation).
Do this for both ends and then remove the Burndy.
You then need to hold each end end of the cable just beyond the join with the plug. Line up in the way it will fit to the units to be connected and, with the dots to the top, "relax" (flop it about) the lead so that there is no tension felt in your wrists as you hold the lead (dots uppermost).
The Burndy can then be offered up to its 2 sockets - being pulled in by the action of the locking collar (yes - do do up, sing it). The internal pins will now slide into their respective sockets - in good alignment and without sideways strain.
Any bend required to fit the cable in a limited space should be made and inch or so away from the immediate point at which it enters the plug.
N.B. - it is also a very good idea to position equipment and racks in such a way that Burndies and leads do NOT press up against the wall behind (or rest on the floor). These can act as a sounding board and feed vibration into the plug/socket and connected equipment - short circuiting the operation of your carefully designed equipment stand.
Why no S-XPS Burndy with the 555PS?
This should be manageable with the help of your retailer.
We have "kits" which we pack with each item. These are picked to minimise unnecessary duplication of leads but also to provide what is necessary to avoid tears before bedtime.
In the case of the 555 the (2 special) Burndies come with the head unit. If we were to provide a Burndy with the 555PS - purchasers of the full combination would have one (useless) spare.
Why don't we box an S-XPS Burndy with 555PSs supplied on their own?
Partly because of the logistics (otherwise - likelihood of a cock-up) and partly because, if we did, there would be two 555PSs and one ..... would cost more.
Probably by the cost of ... an S-XPS Burndy.
The gigantic mental powers of the Sales Department engaged with this conundrum and the present solution IS quite rational.