The 'Starr'-mount did work - in a way. There's a Hetzer site where the experiments for the Starr-mount are covered. As far as I know the main reason for the mount was to get rid of the complicated shock absorbing mechanism in the gun cradle which would have led to an increase in gun output at the factory.
The drawback was a huge shockwave which numbed the gunners hands - it's reported the numbness stayed for several days after each shot. By the end of the war they were trying to decouple the gun laying mechanisms from the shockwave.
My understanding was that they intended to mount the 80mm HLP gun in that model of the Hetzer...They tested a 120mm mortar on a Hetzer and it worked from what I read.
While there’s a bit of conflict re some of the dates and chassis no.s talked about, the Tom Jentz Panzertracts 9 on the "Jagdpanzers", Spielberger's "Leichte Jagdpanzer" and the MBI "Hetzer" book are the best references.
It seems that in May 1944 BMM delivered the single modified, fully converted, Tatra-engined Hetzer “Starr” to Alkett for testing (Fgst.322370), with a further 3 test vehicles mounting the new gun done on normal Hetzers sent on for final completion in September (Fgst. No.s 321679, 321682, & 321683).
While initial range test shoots with them at Jince near Prague had offered promise of success, there were also some worrying teething troubles evident, with the excessive recoil distributed back through the vehicle damaging the elevating gear and/or continually cracking the optics. These bugbears were minimised but never quite overcome before the course of the war ended this program for good.
Nevertheless a trial batch of 10 further “O-series” vehicles mounting the “Starr” gun set-up, but in standard “Hetzer” hulls, were completed alongside the standard ones then on the production line at BMM during December 1944 and January 1945 (5 ea.), thus making a total of 14 completed with the mount by war’s end.
The 14 vehicles made all up, thus took 2 different forms. The first was the radically redesigned body shape with numerous different features including; Tatra Diesel 8 cylinder engine, revised idlers, rear plate, roof and fender layout etc., while the 3 test bed and 10 O-series vehicles were simply standard Hetzer hulls of varying idlers and exhaust configurations running the Praga AC 6 cylinder engine -and simply with the new rigid gun/mantlet and Kugellafette assembly installed into their revised glacis plates.
7 of these and 1 Tatra “Starr” were sent to the Pz.Jg. School in Milovice (Milowitz) in Prague in early 1945 to be used as trainers. They all saw service there in Prague, albeit very briefly in May 1945 with the Prague Garrison and “KG Milowitz” during the uprising.
One other Starr was also stationed at the Berka Proving Grounds with “Pz.Kp. Berka” which was hurriedly formed on March 31st 1945, but was ordered to be blown up on orders direct from Hitler on 31 March 1945.
The fate of the lone Tatra Starr Prototype (Fgst.322370) in is now known thanks to the MBI book. After a long wait for some parts it was finally finished off in March 1945 and initially sent to Milovice for Hitler’s birthday inspections on April 20, but this obviously fell through due to the deteriorating war situation... So it too was then stationed in Prague, Czechoslovakia along with the German Garrison there along with 3 other O-series “Starrs” of the standard design but also sporting the “Starr” guns, and they all fought briefly during the weeklong uprising.
They were first used on the afternoon of May 5th 1945 in Podbaba north of the Vítĕznĕ námĕstí in the Dejvice district, and for the next few days inside the German held area along the west bank of the Vlatava River. On the evening of May 8th the Prague Garrison decided to surrender to the American forces by heading to Rokycany. This lone Tatra “Starr” was left abandoned on the way outside a house in the suburb of Bøevnov where it is seen pictured on p.84 in the MBI Book
Of note re the Tatra one is the lack of the 2 lifting hooks seen on some of the other “Starr” Kugellafettes and mantlets, the “S”-hooks in the front tow eyes, the 6-spoked stamped ribbed idlers, and the late square WZF 2/2 gunsight. While dusty, the striped wavy BMM camo scheme is also discernible - on the skirts in particular.
Post-war, the Czechs still listed 8 "Starrs" as having survived and they were re-armed with the standard guns due to excessive wear on the “Starr” guns and lack of replacement parts. The lone Tatra "Starr" seen abandoned at Bøevnov, was also kept at a training facility for some years before being scrapped.
All up about 10 pics are known to exist of them in service and postwar - apart from the MBI book's collection on pp.84-85 + Front inside cover page, there’s a well-known shot of a 140/1 in a dump with one glimpsed in the background (eg. p.121 Tech Intell 1), and Tomáš Jakl who wrote the MBI chapter.