Simon,
I have not come across a five battalion strong regiment (which in effect would be required to field a 19th company).
Certainly at mobilization in 1914 a handful of regiments had four battalions (RIR 99, RIR 130 and BRIR 4) which would give companies 13-16. In fact as the war progressed, the number of rifle companies decreased, giving a final establishment of 3 rifle companies and one machine gun company per battalion.
Prior to the war, in the late nineteenth century, the Prussian Army had trialed a four battalion establishment when in April 1887, the following 12 Prussian regiments were authorised a fourth battalion in the strength of
four rifle companies each (13., 14., 15., 16.):
13, 14, 16, 17, 18, 39, 40, 53, 65, 80, 83, & 129. In July 1890 this had been utilized to form new regiments.
And again in August 1893 all extant infantry regiments were authorised to form a fourth battalion in the strength of two companies (13. and 14.) Once again these were used to form new regiments in April 1897.
Regards
Glenn