The exact make-up of the British flank battalions at various times during the war, has long been subject to debate. There is a guy called Steve Gilbert who has spent many years researching which companies were where for each campaign; some of the information is in extremely obscure places. Take what is written below as my best estimates based on my own research.
The two Germantown battalions (strength 70 officers and 1,008) were:
1st Bn – 4th, 5th, 10th, 15th, 17th, 22nd, 23rd, 27th, 28th, 33rd, 35th, 38th, 42nd and either 44th Foot or 55th Foot.
2nd Bn – 37th, 40th, 43rd, 44th (or 55th), 45th, 46th, 49th, 52nd, 57th, 63rd, 64th, 1/71st, 2/71st, 1/Marines, 2/Marines.
By the time of Monmouth (victualling strength on the morning of the battle 1,498 all ranks), they were:
1st Bn – 4th, 5th, 10th, 15th, 17th, 22nd, 23rd, 27th, 28th, 33rd, 35th, 38th, 42nd and 55th.
2nd Bn – 37th, 40th, 43rd, 44th, 45th, 46th, 49th, 52nd, 54th, 57th, 63rd, 64th (the 71st and Marines companies had been sent back to NYC and Halifax, respectively – not sure who replaced them, but the 7th and 26th, recently exchanged after their capture in Canada, are prime candidates).