If they are factory Mk IV Improved's then obviously there is a batch of them that were in the transition phase between the Mk IV phasing out, and the Mk IV Improved coming in.
I'm not sure if I can understand the above statement. There are the original format Mk IV's that use a full complement of KEF drivers and then they changed to the first of the "Improveds" which were an Elac / Celestion combination, followed by the second version of "Improveds" with those having the full Elac complement of drivers. I have posted up some images of all 3 variants plus a Dealer / Owner upgraded set (image 4) so that this eliminates any confusion.
The Original RSPM Mk IV using all KEF drivers, comes complete with Metal Stands. (stands not pictured)
The First issue of the
" Improved" RSPM Mk IV variant using the Elac / Celestion Line up of drivers, complete with Timber Stands
( Mk V unofficially )
The Second and Last issued variant of the
"Improved" RSPM Mk IV using the full contingent of Elac only drivers, complete with Timber Stands. Note the totally different Tweeter that replaced the HF1300 with the large face and also the sharper cleaner looking Super Tweeter that replaced the HF2000. ***Please Note that the reticulated Foam inserts are missing from the outlet ports. These all eventually break down and crumble, thus you can see the reminants still attached with traces of adhesive used.
( Mk VI unofficially ) thus why the next variant of the RSPM was the Mk VII (7)
The following image is out of production sequence and of an original RSPM Mk IV with a
dealer fitted "Improved Kit" installed.
Note there is no "Improved" Decal fitted and also this set has the earlier Green mesh in the outlet ports which signifies that this was originally a KEF / Celestion fitted set.
**Note HF2000 Super Tweeter grille is missing. (common issue)
And Lastly,
The RSPM Mk VII (7) Note the change of the Midrange and Tweeter inverted line up and also the face of the Tweeter is different (reduced in size) to the last issued Mk IV
"Improved".
Thank you to member Tor-S for the image used below.