I mentioned the ACS Software stuff a couple of years ago when I joined this forum, and was hoping some of the titles might have surfaced since then. Sadly still only PROGMERGE is present at the excellent ZX81 archive site.
Having an idle few days I wrote this new one, ACSEM, to have the same look and feel as Acsembler. It is more powerful though. A '.P' file is provided along with a binary file which can be loaded using (e.g.) ZXPAND[+]. A description/instructions are also provided as part of the zip file.
Obviously retro addicts use cross-assemblers these days rather than native ones, and indeed ACSEM was written with one. However, using it gives a feel for how things were done back then. It certainly makes it feel like the original wasn't fully lost.