...an indication about a G.61 project appears on 'Gli Aeroplani Italiani 1945-1991', by late Italo de'Marchi.
In the chapter devoted to the G.59 Italo wrote:
"After study, on two P-51 leased from Aeronautica Militare, of the RR Merlin Packard V-1650-7 engine 8as a batch of those less costly engines was available), it (this version of the engine) was installed on a G.59, initially known as G.55AP, but without any flight test. Identified also as G.61, it would have a max speed of 670 km/h..."
In the de'Marche text there is no indication of the G.60.
Both G.60 and G.61 are in "I Caccia della Serie 5 - Parte II -Fiat G.55" by Nino Arena.
About postwar developments of the Centauro Nino wrote:
From the G.55 were developed some experimental versions, some only prototypes and other produced in quantity:
1) G.55AD (or G.48A) with IF Delta RC.40 engine.
2) G.55BBD (G.48B), same but two-seater
3) G.55AM (G.59A) w/ RR Merlin T.24/2
4) G.55BM (G.59B), same but two-seater
5) G.55A/66 (G.60A) with RR Merlin 66 engine
6) G.55B/66 (G.60B), same but two-seater
7) G.55AP (G.61A) with Packard V-1650-7
8) G.55BP (G.61B), same but two-seater
The G.61, continued Nino, was built only in the single-seat prototype form, whilst the G.60 was redesignated G.59-2 and received the Merlin 500/20.
I think all the above is very interesting, but I cannot endorse it. I hope to find the time of dig in the papers of an old interview with Prof. Gabrielli...
Nico