[Groff] Desable and enable page numbers with ms

[trebol]

Hello all,
I would like to start a chapter without page number, simply starting the
paragraph a few lines below normal, and then restore the page numbers.
The only way I have done it is with:

.ds CH
.de CHA
.   OH ''''
.   EH ''''
.   bp
    \&
.   sp 8v
.   OH '''%'
.   EH '%'''
..
.
.CHA
.so ch_1.ms
.CHA
.so ch_2.ms

There is a simple way to disable and enable the page numbers, without
touch the headers?  I'm new in troff/groff, maybe I'm doing a useless mess.
Also, I found the PN register in utp, but neither
in the groff_ms page or in the info manual.  There are other interesting
undocumented registers?

Thanks to all.