As you've found out, the Intrinsics manual is misleading on this point.
You can find mention of this behavior documented in the manual

  Interprocess Communication Programmer's Guide (p/n 32650-90019 )
  Appendix A - Features of Intrinsics for Message Files, page A-2.

So, the default behavior is to open as Fixed. I never gave much thought
to the performance implications until reading this thread. (I learn
something new *every* day).

Tony Summers wrote:
>
> A big thank you to Tad Bochan, Stan Sieler and Gavin Scott.
>
> In summary.  I was writing to a (pre-built) variable length message file
> and then reading it back, but the records were being returned as fixed length
> records.
>
> The problem is that you have to OPEN the message file for READ setting
> the Foption bits for RECORD FORMAT = VARIABLE LENGTH.
>
> Thus, as Stan stated open it as Fopen(f,%107), but since
> I use HPFOPEN,  this translated to Item Number = 6 and item value = 1.
>
> What's confusing is that my manual states that this Foption setting
> is only applicable at file creation.     Mmmmmmmm !
>
> Many thanks again.