[tankbarrell]

I found a reference to that in an official history.
In The development of artillery tactics and equipment, published in 1951 by the War Office, it states that '100 old 3" 20 cwt guns were collected, provided with a special 12 1/2 lb. shot and mounted half in Churchill tanks and half on 17-pr carriages, the production of which was in advance of that of the guns. The Churchill mounting was handicapped by its small internal traverse-only 7 degrees- and the guns so mounted were allotted for home defence only. Of the remaining 50 on 17-pr carriages, 25 were allotted to the Middle East and 25 to Home Forces.'

The reference to production of 17-pr carriages being in advance to that of the guns rather contradicts the idea that 17/25 pr came about due to a shortage of carriages for the 17-pr.